The Problem of Symmetrical Threats*[author’s name redacted]

Words (including footnotes): 11, 453

ABSTRACT: According to mainstream theories of defensive

killing, a person becomes liable to such killing by posing

an impermissible threat of death or serious injury to others

which, under certain conditions, gives others a permission

to kill that person in self- or other-defense. I argue that

this account of the relation between the impermissibility of

a person’s threat, her liability-status, and the

permissibility of one’s defensive response leads to

contradiction given several plausible assumptions about

symmetrically situated attackers.

1. The Problem

Most theories of self-defense recognize at least one

non-consequentialist justification for defensive

killing, which appeals to the claim that an attacker

has acted in such a way that killing him wouldn’t wrong

1

him or violate his right not to be killed. Following

Jeff McMahan, I shall say that such a person is liable to

defensive killing.1 According to the most popular

account of defensive killing in the literature, one

becomes liable to such killing by threatening

impermissibly to violate another’s right not to be

killed or seriously injured. One thereby forfeits one’s

right not to be killed, making it permissible for

others to kill one provided that this is both necessary

for stopping one’s threat and proportionate i.e. it won’t

violate anyone else’s rights or produce any bad effect

that outweighs the good achieved by stopping one’s

threat. If one isn’t liable to defensive killing then

one retains one’s right not to be killed and therefore

killing one is prima facie impermissible. Call this the

Standard View of the morality of defensive killing.

1 *For helpful comments, I am grateful to [names have been

redacted for blind review].

“The Basis of Moral Liability to Defensive Killing,”

Philosophical Issues 15 (2005): 386-405, p. 386

2

The most influential version of the Standard View is

held by Judith Jarvis Thomson.2 On this view one can

threaten to violate another’s rights without either

intending to do so, or exercising one’s capacity for

moral agency. Suppose that, through no fault of his

own, an unconscious man will soon land on you and crush

you to death. The only way for you to prevent this is

by vaporizing him with your laser gun (otherwise, he’ll

survive the fall).3 On Thomson’s view, the unconscious

man would violate your right not to be killed if he

landed on you, and is therefore liable to defensive

killing. In contrast to Thomson, some defenders of the

Standard View claim that liability to defensive killing

requires that one meet some subjective criterion, such

as culpability or minimal moral responsibility, for an

act by which one threatens others.4

2 See Thomson, Judith Jarvis, “Self-Defense,” Philosophy & Public

Affairs 20 (1991): 283-310.

3 Ibid., p. 287.

4 See, e.g., Otsuka, Michael, “Killing the Innocent in Self-

Defense,” 23 (1994): 74-94, McMahan, “The Basis of Moral Liability

3

I will argue that all versions of the Standard

View are undermined by examples such as the following:

Symmetry Case 1. Smith maliciously intends to kill

his archrival, Jones, by shooting him with a

sniper rifle. He is unaware that Jones, acting

with a similar intention, plans to assassinate

Smith in precisely the same manner. As Smith peers

through the scope of his rifle, he sees Jones

looking back at him through the scope of a similar

rifle. Still acting with malicious intent, but now

realizing that his own life is under threat, each

to Defensive Killing”, Lazar, Seth, “Responsibility, Risk, and

Killing in Self-Defense,” Ethics 119 (2009): 699-728, and Quong,

Jonathan, “Killing in Self-Defense,” Ethics 119 (2009): 507-537 as

well as “Liability to Defensive Harm,” Philosophy & Public Affairs 40

(2012): 45-77. Most writers also reject Thomson’s claim that one

can violate another’s right not to be killed merely by falling on

this person. It is widely held that one must act in one’s capacity

as an agent in order to violate a right.

4

rushes to kill the other before the other can kill

him. They fire simultaneously.5

In Symmetry Case 1 Smith and Jones engage in what I will

call symmetrical self-defense. The term ‘symmetrical self-

defense’ has been used elsewhere to describe cases in

which two parties justifiably threaten each other, and

neither seems liable to defensive killing.6 I will use

it to refer to any instance of defense involving two

persons, A and B, (symmetrical self-defenders) who

threaten each other, in which (1) Neither A nor B

threatened first, (2) A will kill B if B doesn’t kill A

and vice versa, (3) A is the only individual B

5 An actual case of this kind occurred during the Vietnam

war, in which an American marine, Carlos Hathcock, shot and killed

a Vietnamese sniper known as “the cobra” through the scope of the

latter’s own rifle. (Charles Henderson, Marine Sniper: 93 Confirmed Kills

(Berkeley Books, 2001)). I am grateful to [redacted] for bringing

this case to my attention.

6 See e.g. Mapel, David. “Moral Liability to Defensive

Killing and Symmetrical Self-Defense,” The Journal of Political Philosophy

(2009): 1-20.

5

threatens and vice versa, and (4) other morally

relevant factors such as the causal mechanisms through

which A’s and B’s threats are carried out, their

reasons for threatening, and the intentions, motives,

etc., with which they threaten are the same in every

respect that is relevant for determining the

permissibility of their acts as well as the harm to

which they are liable. Condition (2) ensures that each

defender’s threat is necessary for stopping the other’s

threat, while condition (3) seems to ensure that if

each defender is liable to defensive killing, then the

act by which each threatens the other is proportionate;

neither will violate anyone else’s rights or produce

any bad effect that outweighs the good of stopping the

other’s threat.

The Standard View assumes that the permissibility of

defensive killing depends at least partially on the

impermissibility of a liable attacker’s act; it

therefore assumes a deontic asymmetry between a liable

attacker and one who permissibly threatens to kill that

6

attacker in self- or other-defense. But in symmetrical

self-defense there is no such asymmetry. Thus, the

Standard View’s assumption of deontic asymmetry is false.

I call this the problem of symmetrical threats.

In Section 2, I argue that the problem of

symmetrical threats can be understood in terms of an

inconsistency between the Standard View and several

plausible assumptions about symmetrical self-defenders,

which I shall spell out. In Section 3, I consider and

reject several attempts to solve the problem of

symmetrical threats while maintaining the Standard View’s

required asymmetry. In section 4, I provide a summary

of the argument, and briefly offer some suggestions

about how to proceed if we abandon the Standard View’s

assumption of deontic asymmetry.

2. Dissecting the Problem

Several authors have discussed cases involving

symmetrically situated individuals who threaten each

other simultaneously; however none has fully

7

appreciated the extent of the problems that such cases

raise for the morality of defensive killing. Michael

Otsuka discusses an example in which persons A and B

are falling toward each other and will fatally collide

unless one vaporizes the other with his ray gun (the

one who vaporizes the other will land safely in a

net).7 Otsuka points out that on Thomson’s version of

the Standard View (i) whether A may vaporize B, depends

on (ii) whether B impermissibly threatens to violate

A’s right not to be killed; but A’s possessing such a

right depends on (iii) whether A impermissibly

threatens to violate B’s right not to be killed, which

depends on (iv) whether B possesses such a right,

which, finally, depends on (v) whether B impermissibly

threatens to violate A’s right not to be killed. Thus,

whether each person impermissibly threatens to violate

the other’s right depends on whether the other

impermissibly threatens to violate his right. This

7 See Otsuka, “Killing the Innocent in Self-Defense,” n. 16.

Otsuka attributes the example to Gary Gleb.

8

symmetrical dependence makes it “difficult to determine

… whether either possesses a right that the other will

violate, and hence whether either may shoot in self-

defense.”8

In fact, as Russell Christopher has pointed out,

Thomson’s account seems internally inconsistent. To

show this, Christopher presents an example involving

multiple persons who threaten each other in a circular

pattern i.e. A threatens B, who threatens C, who in

turn threatens A.9 If A has the right not to be killed,

then in threatening A, C is liable, and so forfeits his

8 Ibid.

9 See Christopher, Russell, “Self-Defense and Defense of

Others,” Philosophy & Public Affairs (1997):137-38. Christopher also

briefly considers a case similar to symmetrical self-defense

involving only two agents. He argues that, since there is no

significant basis to distinguish between the two agents

“objectivist” approaches—those that do not include a subjective

element of the agent’s mental states among their criteria for

liability—entail that each agent’s act is both justified and

unjustified, a contradiction. Technically, that an act is both

justified and unjustified does not amount to a contradiction.

9

right not to be killed. Since C lacks the right not to

be killed, B’s threat to C is permissible, and so B

isn’t liable; he retains the right not to be killed.

But if B has the right not to be killed, then according

to Thomson’s theory, the fact that A threatens B

entails that A lacks the right not to be killed.

Thomson’s theory therefore entails both that A has the

right not to be killed and that A doesn’t have the

right not to be killed, which is clearly a

contradiction.

According to Christopher, the problem with

Thomson’s theory is that it fails to recognize the

moral relevance of subjective features of an agent’s

mental states, such as his intentions, for determining

the permissibility of the agent’s act. I will argue

that the threat of contradiction arises even for views

that incorporate such subjective criteria, where this

includes not only views on which intention is relevant

to liability and permissibility, but also those on

which liability is a function of what it is

10

impermissible to do relative to one’s evidence or one’s

beliefs. Thus, failure to incorporate subjective

criteria isn’t the source of the problem. It is rather

the moral asymmetry implied by the Standard View between

a liable attacker and one who permissibly defends

against his attack.

I will treat the following two claims as

preliminary conditions of adequacy that a plausible

solution to the problem of symmetrical threats must

satisfy:

Symmetry: If A and B are symmetrical self-

defenders then the act by which A threatens B is

permissible if and only if the act by which B

threatens A is permissible, and A is liable if and

only if B is.

AndDeontic Monoletheism (DM): An act is permissible only

if it isn’t impermissible.

11

DM is a plausible axiom of deontic logic that hardly

requires defense. It is widely endorsed (often

implicitly), even by those who countenance moral

dilemmas i.e. situations in which an agent will act

wrongly no matter what he does.

Symmetry, unlike DM, is not unassailable; however it

is quite plausible given that symmetrical self-

defenders are morally similar in every respect that is

relevant for determining the permissibility of their

acts. How similar must the defenders be to satisfy

Symmetry? Start with an easy case; suppose the

defenders are atom-for-atom duplicates, have exactly

similar surroundings, and mirror each other’s bodily

movements perfectly (although there is still some

objective chance of one’s surviving and killing the

other). Symmetry seems obviously true of this pair of

defenders. I don’t think that Symmetry plausibly

requires perfect natural similarity or even perfect

moral similarity. It could apply to two defenders even

if one was, for example, more culpable for his act than

12

the other. But whether this is true has no bearing on

Symmetry’s plausibility; it matters only for determining

the range of actual and hypothetical cases in which

Symmetry applies. For the sake of simplicity, I will

consider cases in which those who threaten each other

are perfectly naturally similar.

In addition to Symmetry and DM, I will assume that

there are possible cases in which each of the following

claims is true:

(a) There are at least two persons, A and B, who are symmetrical self-defenders, each of whom satisfies the subjective criteria necessary forliability,

(b) each of A’s and B’s individual acts is eitherpermissible or impermissible,

and

(c) neither A nor B has a non-liability-based permission to threaten anyone.

Assumption (a) is clearly true in Symmetry Case 1;

Smith and Jones are by definition symmetrical self-

defenders, since they satisfy the four relevant

conditions. That is, (1) neither is the initial

13

threatener, (2) each will kill the other if the other

doesn’t kill him, (3) neither threatens a third party,

and (4) other morally relevant factors are the same in

every respect that is relevant for determining the

permissibility of their acts as well as the harm to

which they are liable. Moreover, since Smith and Jones

are both fully culpably responsible for their acts,

they meet even the most conservative subjective

criteria of liability.

(b) also seems true in this case. If (b) is false,

then Smith’s and Jones’s acts are neither permissible

nor impermissible. This may be true of certain acts.

But it’s difficult to see why it would be true of

Smith’s and Jones’s acts. Clearly, each of their acts

would be either permissible or impermissible on the

condition that one of them was the initial threatener;

but the deontic laws of the universe shouldn’t collapse

merely because their threats are initiated

simultaneously. Thus, it seems, there is no principled

way of rejecting (b).

14

Assumption (c) is also difficult to deny. Those

who reject (c) must explain why Smith and Jones are

permitted to threaten to kill each other. The

literature on the ethics of killing is filled with

discussion of many alleged kinds of permission to kill.

These include permissions based on the explicit or

implicit consent of the person to be killed, desert,

enforceable duties, the redirection (as opposed to the

creation) of a threat, and various kinds of lesser-evil

justification. But neither Smith nor Jones has any such

permission.

One possibility is that each has an agent-relative

permission to threaten the other. But such permissions

are not plausibly unconditional: there are conditions

for forfeiting them. I will argue in section 3 that

such conditions cannot plausibly be established without

running back into the problem of symmetrical threats.

Thus, appealing to agent-relative permissions merely

pushes the problem around.

15

I will show that Symmetry, DM, (a), (b), (c), and

the Standard View are jointly incompatible. For this

purpose, I must provide a more precise formulation of

the Standard View. This view has two major components.

The first is a criterion of liability to defensive

killing, which I will call

Liability Condition: Person x is liable to defensive

killing for threatening to kill or seriously

injure person y if and only if (i) the act by

which x threatens y is impermissible and (ii) x

meets whatever subjective criteria (e.g.

culpability, moral responsibility, etc.) are

necessary for liability.

The second component determines when one has a

liability-based permission to kill a person. I call

this

Defensive Condition: If person x doesn’t have a non-

liability-based permission for threatening to kill

16

person y, then the act by which x threatens y is

permissible if and only if (iii) y is liable to

defensive killing and (iv) x’s act is both

necessary and proportionate.

Now suppose for reductio:

1. The act by which Smith threatens Jones is

impermissible.

Then,2. Smith is liable to defensive killing. (by 1, (a), Liability Condition Modus Ponens)

Therefore,

3. The act by which Jones threatens Smith is permissible. (by 2, (a), (c), Defensive Condition Modus Ponens)Therefore, 4. The act by which Smith threatens Jones is permissible.

(by 3, (a), Symmetry Modus Ponens)

Therefore,

5. The act by which Smith threatens Jones isn’t impermissible. (by 4, DM Modus Ponens) Contradiction!

Next, suppose:

17

6. The act by which Smith threatens Jones is permissible. Then,

7. The act by which Jones threatens Smith is permissible. (by 6, (a), Symmetry Modus Ponens)

Therefore,

8. The act by which Jones threatens Smith isn’t impermissible. (by 7, DM Modus Ponens)

Therefore,

9. Jones isn’t liable to defensive killing. (by 8,Liability Condition Modus Ponens)

Therefore,

10. The act by which Smith threatens Jones isn’t permissible. (by 9, (a), (c), Defensive Condition Modus Ponens) Contradiction!

Since the premise that Smith’s act is impermissible and

the premise that Smith’s act is permissible both lead

to contradiction, Smith’s act can be neither

permissible nor impermissible. But this contradicts

assumption (b), which states that each defender’s act

is either permissible or impermissible.

18

This completes the proof; (a), (b), (c), Symmetry,

DM, and the Standard View are jointly inconsistent. The

problem of symmetrical threats can be understood as the

pressure this exerts on us to reject the Standard View,

or else emend it in some non-ad hoc way. We will now

consider several different ways of resolving the

inconsistency while preserving the Standard View’s

assumption of deontic asymmetry. These include:

emending the Standard View so as to render it compatible

with the other five assumptions, treating symmetrical

self-defense as a special exception to the Standard

View; rejecting Symmetry, and rejecting assumption (c)

i.e. the assumption that neither defender has a non-

liability-based permission to threaten the other.

3. Six Proposals

3.1 Emending the Standard View by Adding a ‘right intention’ clause to

Liability Condition

One way of emending the Standard View appeals to the

relevance of intention to permissibility. There may be

19

cases in which one has a justification for acting a

certain way but acts impermissibly in virtue of

possessing a wrongful intention. For example, consider

Bad Trolley Driver. The driver of a runaway trolley

turns it away from five innocent people towards

one, thereby saving the five and killing the one,

but only in order to kill the one and not to save

the five.

If intentions are relevant to the permissibility

of individual act-tokens, then although the act type

‘turning the trolley from the five to the one’ may be

permissible, in this case the individual token of the

permitted type may be impermissible for the reason that

the driver intends to kill the one. Similarly, one

might argue that in Symmetry Case 1, although Smith and

Jones are liable to defensive killing and therefore

have liability-based justifications for threatening to

kill each other, they nevertheless act impermissibly

because they act with wrongful intentions. The

20

inconsistency proved earlier is avoided by adding a

‘right intention’ clause to Defensive Condition alongside

its necessity and proportionality clauses. Because

neither Smith nor Jones satisfies the ‘right intention’

clause, neither satisfies Defensive Condition; thus,

although each has a liability-based justification for

threatening the other, this justification isn’t

sufficient to make his act permissible. On this

proposal Smith and Jones are both liable and both act

impermissibly.10 This is compatible with Symmetry, DM,

and assumptions (a), (b), and (c).

However the proposal is ineffective, for consider

Symmetry Case 2. Similar to Case 1, but with the

following backstory: Each defender falsely

believes that the other is maliciously attempting

to kill him, and that he can defend his life only

10 Note that the defenders can fail to satisfy the ‘right

intention’ clause even if, in addition to wrongfully intending to

kill the other, each also intends to save himself from the other’s

threat.

21

by killing the other. Each attacks the other

without malice, acting only in self-defense.

However, each knows there is a chance that the

other is innocent and that his belief about the

other’s intentions might be mistaken. Moreover

each knowingly and culpably ignores crucial

evidence that bears on the permissibility of his

act. If either defender paid attention to this

evidence, he would have realized that the other

had no prior intention of killing him, and the

conflict would be avoided.

In Symmetry Case 2, because Smith and Jones are culpably

responsible for their threats, each meets the necessary

subjective criteria for liability. However, since

neither acts with a wrongful intention, each satisfies

the ‘right intention’ clause of Defensive Condition. Thus,

it seems there are cases of symmetrical self-defense

that can generate the problematic inconsistency even

22

once the Standard View has been emended to reflect the

alleged relevance of intention to permissibility.

3.2 Appealing to an evidence-relative account of liability

Another way of altering the Standard View appeals to an

evidence-relative account of liability. Here some

clarifications will be helpful. Following Derek Parfit,

we can distinguish three different standards of

evaluation of an act. An act is

permissible/impermissible in the fact-relative sense (or

fact-relative permissible/impermissible) when its

permissibility/impermissibility is implied by the most

general true moral principles together with the non-

moral facts.11 One’s act is permissible/impermissible 11 My understanding of fact-relative permissibility differs

somewhat from Parfit’s. According to Parfit, an act is

permissible/impermissible in the fact relative sense whenever the

act would be permissible/impermissible if the agent who performed

it knew all the facts. But this is inadequate. There could be

cases in which, if the agent knew all the facts, this would change

the permissibility of the act in question. Suppose, for example,

that when I purchase a candy bar from my local convenience store,

23

in the belief-relative sense (or belief-relative

permissible/impermissible) if one’s beliefs together

with the most general true moral principles (whether or

not one believes them) jointly entail that one’s act is

fact-relative permissible/impermissible. Finally, one’s

act is permissible/impermissible in the evidence-relative

sense (or evidence-relative permissible/impermissible)

if one’s evidence indicates that one’s act is

permissible/impermissible. By “one’s evidence indicates

that p,” I mean that whatever one’s evidence gives one

decisive reason to believe together with the most

general true moral principles jointly entail p, whether

or not these moral principles can be inferred on the

an evil demon will kill 10 innocent people if I also know all the

facts. If I don’t know all the facts, then the demon won’t harm

anyone. In this case, Parfit’s account seems to entail that it is

fact-relative impermissible for me to purchase the candy bar

since, if I knew all the facts, this act wouldn’t be permissible.

But this is clearly false. My understanding of fact-relative

impermissibility avoids this result, since it doesn’t depend on

what the agent would know in some possible scenario.

24

basis of one’s evidence. Thus, for one’s evidence to

indicate that some act is permissible or that some

person is liable to defensive killing, it needn’t be

the case that one is able to infer the true principles

of permissible defensive killing, or the principles of

liability to defensive killing, on the basis of one’s

evidence and then apply these principles to one’s

current circumstances. It is enough that what one’s

evidence gives one decisive reason to believe, together

with the true general principles (whatever they are)

jointly entail that some act is permissible or that

some person is liable to defensive killing.

The proposal we will consider involves replacing

the Standard View’s Liability Condition with

Evidence-Relative Condition: Person x is liable to

defensive killing for threatening to kill or

seriously injure person y if and only if (i) the

act by which x threatens y is evidence-relative

impermissible and (ii) x meets whatever subjective

25

criteria (e.g. culpability, moral responsibility,

etc.) are relevant for liability.

Call the conjunction of Evidence-Relative Condition and the

Standard View’s Defensive Condition the Evidence-Relative Account of

Liability to Defensive killing (ERA).

ERA seems promising as a solution to the problem

of symmetrical threats because the deontic asymmetry it

implies is between two different kinds of deontic status of

an act. The kind of impermissibility that matters for

liability is evidence-relative, but the permission to

engage in defensive killing that is grounded in facts

about liability is fact-relative. To see how this makes a

difference, consider Symmetry Case 2 in which symmetrical

self-defenders Smith and Jones are mistaken attackers.

Suppose for the sake of argument that the act by which

each threatens the other is evidence-relative

impermissible. That is, suppose that what each

defender’s evidence gives him decisive reason to

believe, together with the relevant moral principles,

jointly entail that the act by which each threatens the26

other is fact-relative impermissible. Then according to

Evidence-Relative Condition, both are liable to defensive

killing. Together with Defensive Condition this entails

that both act permissibly in the fact-relative sense.

Alternatively, suppose that each defender’s act is

evidence-relative permissible. Then according to

Evidence-Relative Condition neither is liable to

defensive killing. Together with Defensive Condition this

entails that both act impermissibly in the fact-relative sense.

Thus, according to ERA, either both defenders are

liable to defensive killing and each acts permissibly

in the fact-relative sense, or neither is liable to

defensive killing and each acts impermissibly in the

fact-relative sense. This is consistent with Symmetry,

since each defender acts permissibly if and only if the

other acts permissibly, and it is consistent with DM,

(a), (b), and (c) as well. ERA appears to provide an

elegant solution to the problem of symmetrical threats.

However, a problem arises for ERA when one

defender’s evidence supports the belief that whatever

27

his evidence indicates about the permissibility of his

act, the other defender’s evidence indicates about the

permissibility of the other’s act. This could happen if

each defender’s evidence indicated that both had

qualitatively similar evidence. For example, consider

Symmetry Case 1 in which Smith and Jones maliciously

intend to kill each other. Suppose we stipulate that

each has made many such attempts in the past, and that

each is fully aware that the other intends to kill him.

Moreover, suppose that as each sees the other through

the scope of his rifle, each sees a startled look

appear on the other’s face as he fumbles nervously to

remove the safety on his rifle. Each therefore observes

that the other is at least partially motivated by self-

preservation; and each sees that the other observes

this about him. But because each is aware of the

other’s murderous intentions, each also becomes aware

that both have been plotting to kill each other, and

that they are also partially motivated by malice. Thus,

each has evidence which indicates that the morally

28

relevant features of each defender’s act are the same.

Let us stipulate

Evidential Symmetry: Smith’s evidence indicates that the act by which

Smith threatens Jones is fact-relative

permissible/impermissible, and that Smith meets

the necessary subjective criteria for liability

if and only if

Smith’s evidence also indicates the following:

Jones’s evidence indicates that the act by which

Jones threatens Smith is fact-relative

permissible/impermissible, and that Jones meets

the subjective criteria necessary for liability.

Let us also assume

(a’) Smith’s evidence indicates that if Jones is liable to defensive killing then Smith’s act is necessary and proportionate.

and(b’) Each defender’s act is either (evidence-relative) permissible or (evidence-relative) impermissible.

29

If ERA is true then it either is, or follows from, the

most general true moral principles. However, given this

seemingly trivial observation, Evidential Symmetry, DM,

(a’), (b’), and ERA are jointly inconsistent. (The

formal proof of this is in the Appendix.) Intuitively,

the inconsistency arises as follows: If Smith’s

evidence indicates that his act is fact-relative

permissible (i.e. Smith’s act is evidence-relative

permissible) and that he meets the subjective criteria

necessary for liability, then Evidential Symmetry implies

that Smith’s evidence indicates the following: Jones’s

evidence indicates that Jones’s act is fact-relative

permissible and Jones meets the criteria necessary for

liability. But if what Smith’s evidence indicates about

Jones is true, then given ERA and our other background

assumptions, Jones isn’t liable to defensive killing.

Thus, Smith’s evidence indicates that he (Smith)

threatens a non-liable person and that his act is

therefore fact-relative impermissible. Thus, Smith’s

evidence indicates that his act isn’t fact-relative

30

permissible, which contradicts our initial assumption.

On the other hand, if Smith’s evidence indicates that

his act is fact-relative impermissible (i.e. his act is

evidence-relative impermissible) and that he meets the

necessary criteria for liability, then given ERA and

our other background assumptions, Smith is liable to

defensive killing, and so the act by which Jones

threatens to kill Smith is fact-relative permissible.

Given Evidential Symmetry, Smith’s evidence indicates that

Smith’s act is fact-relative permissible, and therefore

that his act isn’t fact-relative impermissible. In

other words, Smith’s act isn’t evidence-relative

impermissible. But this contradicts our initial

assumption.

Since the premise that Smith’s act is evidence-

relative permissible and the premise that his act is

evidence-relative impermissible both lead to

contradiction, it seems Smith’s act can be neither

evidence-relative permissible nor evidence-relative

impermissible. But this contradicts (b’), which states

31

that each defender’s act is either evidence-relative

permissible or evidence-relative impermissible.

This spells disaster for ERA. Since (a’) and

Evidential Symmetry can be stipulated, defenders of ERA

must reject either DM or (b’). But these claims are

eminently plausible, and there is no apparent reason

for denying either of them. Thus, it seems, we can’t

plausibly appeal to an evidence-relative account of

liability in order to resolve the problem of

symmetrical threats.

3.3 Approximating the Ideal Distribution of Harm

Jeff McMahan has recently argued that the Standard View is

most plausibly understood as rooted in more fundamental

considerations about the just distribution of harm when

harm will inevitably occur.12 In such cases, ideally,

anyone who bears at least some moral responsibility for

the threat of the inevitable harm will incur a

12 See McMahan’s “Who is Morally Liable to be Killed in War,”

Analysis 71 (2011): 511-12 and 544-59.

32

percentage of it that is proportionate to the degree to

which he is responsible for causing it. To put the

point slightly differently, the ideally just

distribution is one that divides an unavoidable harm

between agents in perfect proportion to the share of

responsibility each bears for the existence of the

harm.13 For example, if a rock will fall on us, and I

bear 70% of the responsibility for this threat and you

bear 30%, then assuming the harm can only be divided

between us, and can’t be reduced or avoided entirely,

the ideally just distribution would be achieved by one

of us shooting the rock with his trusty laser gun in

such a way that 30% of it falls on you and 70% on me

(suppose the amount of harm each of us incurs is

proportionate to the percentage of the rock’s mass that

falls on us.)

But in most if not all cases the ideally just

distribution is impossible to achieve. In these less 13 I am grateful to [redacted], who suggested this

formulation of the just distribution view, as well as the example

of the falling rock.

33

than ideal cases, the inevitable harm ought to be

distributed in whatever way best approximates the ideally

just distribution. Thus, in non-ideal cases, one is

liable to whatever harm the non-ideally just

distribution determines. Call this The Just Distribution View.

To illustrate, suppose the harm that will be caused by

the rock cannot be divided but must be distributed 100%

to me or 100% to you. Then justice dictates that I am

liable to the whole 100%, since this is what best

approximates the ideally just distribution. But suppose

we are equally responsible for the threat of harm that

will be caused by the rock. Then the best approximation

of the ideally just distribution would be a fair coin

toss. Whoever loses the toss is liable to 100% of the

harm.

Now consider an analogous case of symmetrical

self-defense:

Symmetry Case 3. A trolley is rolling along track A,

and will soon shift randomly onto one of two branch

34

tracks, B and C, at a fork down the line from the

its current position. A randomized track-shifting

mechanism at the intersection of tracks A, B, and C

ensures this. Brown is trapped on track B, and Green

is trapped on track C; each is in danger of being

killed by the trolley. Each bears an equal share of

responsibility for setting the trolley in motion.

Moreover, each has a remote-control device which, if

activated, will cause the track-shifting mechanism

to force the trolley onto the other man’s track

where it will run him over. If the remote-control

devices are activated simultaneously, the causal

sequences initiated by each device will create

opposing forces that will randomly shift the trolley

onto one of the two tracks. In that case, there will

be a 50% chance of the trolley’s going onto track A,

and a 50% chance of its going onto track B.

At time t, Brown and Green activate their

devices. If Brown hadn’t activated his device at t,

he certainly would have died as a result of Green

35

activating his device at t, as this would haveforced

the trolley onto Brown’s track. Similarly, if Green

hadn’t activated his device at t, he certainly would

have died as a result of Brown activating his device

at t, which would have forced the trolley onto

Green’s track. Moreover, each man certainly would

have lived had the other not activated his device at

t since, in that case, he would have activated his

own device at t, thereby eliminating any chance at t

of the trolley going onto his track.

In Symmetry Case 3 there is an unavoidable and

indivisible harm—one death—that needs to be distributed

either to Brown or to Green. Since each bears 50% of

the responsibility for this inevitable harm, the

ideally just distribution would be each suffering half

of the badness of death. Since the badness of death

can’t be divided in this case, the next best

approximation is a fair coin toss, as it was in the

case of the falling rock.

36

But suppose a fair coin toss is impossible.

Perhaps there is a barrier separating the two trolley

tracks, making it impossible for both men to confirm

the results of the toss. Still there seems to be a

reasonably good proxy, namely, letting events take

their course. This seems plausible since each defender

is identically situated and each has an equal chance of

surviving. The fairness of their contest therefore

seems a sufficiently good distributive procedure for

determining liability. Whoever dies loses the “coin

toss” and is therefore liable; whoever survives wins

the “coin toss” and escapes liability.

Since it entails that only one of the defenders in

a case of symmetrical self-defense could be liable, the

Just Distribution View violates Symmetry. However, we would be

too hasty if we rejected it for this reason. This is

because the Just Distribution View offers a non-ad hoc

explanation of Symmetry’s failure. Specifically, it

acknowledges that there is ex-ante moral symmetry

between symmetrical self-defenders, but it denies that

37

such ex-ante symmetry entails symmetry with respect to

liability, since only one defender could possibly lose

the “coin toss.”14 This is what we should expect on the

assumption that liability is a function of

approximating the ideally just distribution of harm.

However, the Just Distribution View’s treatment of

symmetrical self-defense is problematic for other

reasons. According to this view, the basis of liability

is responsibility for an inevitable harm, which must be

realized either as Brown’s death or as Green’s death.

But one’s responsibility for the risk of one’s own

death doesn’t plausibly make one liable to be killed. For

example, law enforcement officers wouldn’t be justified

in gunning down a person in order to defend him from

his own suicide threat. So it seems the only threat for

which either defender could plausibly be liable is the

threat he poses to the other defender. However, if one

defender kills the other, then according to the Just

Distribution View, the former won’t be liable. Suppose Brown

14 I owe this point to [redacted].

38

kills Green. Then having lost the “coin toss,” Green is

liable to be killed and Brown isn’t. Similarly, if

Green kills Brown, then Brown is liable and Green

isn’t. Thus, it seems no one is liable for the

inevitability of the harm because either (i) we’re not

liable for harms we might cause to ourselves, or (ii)

the inevitable harm went to someone who was liable to

suffer it.

One possible response to this would be that if

Brown dies, then he is liable for the threat of what

would have been Green’s death, even though Brown would

not have been liable if in fact he had killed Green. In

other words, although Brown is liable for a threat of

possible harm, it is impossible for him to be liable for

any harm that obtains. This is bizarre.

A more serious problem for the Just Distribution View

emerges when we consider the relationship between

liability and permissibility. Suppose that Brown is

killed by Green and is therefore liable to be killed.

Does Brown’s being liable make the act by which he 39

threatened Green impermissible ex-post? Does the fact

that Green isn’t liable make the act by which he

threatens Brown permissible ex-post? Suppose that Brown,

guided by his conscience, decides at the last moment

not to press the button on his remote control device,

and that Green presses the button on his device,

thereby killing Brown. If the defender of the Just

Distribution View wants to link liability and

permissibility in the standard way, she must say that

in this scenario, Brown’s threat is impermissible and

Green’s isn’t, even though both threats are similar in

almost every way, and Green succeeds in killing Brown.

Moreover, our defender of the Just Distribution View must be

willing to say that if the possible harm in virtue of

which Brown’s threat is impermissible had actually

obtained, Brown's threat would have been permissible.

This is bizarre. Usually, the impermissibility of an

unsuccessful attempt to φ is explained by the

impermissibility of a successful attempt to φ. There

may be cases in which an unsuccessful attempt to harm a

40

person is impermissible where succeeding would have

been permissible. For example, perhaps a successful

attempt to harm someone would avert a disaster, and

would be permissible for that reason, whereas an

unsuccessful attempt would merely cause unnecessary

lesser harms to others. But in this case, there are no

other weird causal effects of the unsuccessful attempt

that help to explain why it is impermissible when a

successful attempt would have been permissible.

Finally, even if the Just Distribution View could be

revised so as to avoid these problems, it still

wouldn’t tell us anything about cases of symmetrical

self-defense in which neither defender is morally

responsible for its being inevitable that someone will

be harmed. For example, consider

Symmetry Case 4. The same as Symmetry Case 3, except

that neither Brown nor Green is morally responsible

for setting the trolley in motion, or for either’s

being stuck on the tracks.

41

In Symmetry Case 4, although neither defender is

responsible for its being inevitable that someone will

die, each poses a deadly threat to the other when he

activates his remote control device. As in Symmetry Case

3, if one hadn’t activated his device, there would have

been no chance of the other’s dying.

One might argue that Symmetry Case 4 isn’t an

instance of symmetrical self-defense because neither

defender is the ultimate source of the threat; instead,

one might claim, when each activates his device, he

merely attempts to redirect a previously existing threat.

But although there may be an important difference

between creating a new threat and redirecting a threat,

if this objection presupposes that one’s merely

redirecting a threat precludes one’s posing a threat, then

it is clearly false. It is obvious that one can

threaten a person by turning a speeding trolley towards

her, even if the trolley was already in motion and so

was already threatening her to some extent, and even if

42

one isn’t morally responsible for setting the trolley

in motion.

Another reason one might deny that Symmetry Case 4

is an instance of symmetrical self-defense is that,

because each defender’s activating his device at t

creates only a 50% chance of the other’s dying, and

this chance is no greater than it would have been had

neither activated his device at t, neither actually

raises the chance of the other’s dying. Thus, neither

actually threatens the other. But this falsely assumes

that how much each defender’s chance of dying at t is

raised by the activation of the other’s device can be

determined by comparing the possible world in which

each device is activated at t (call this the case world)

with a possible world in which neither device is

activated at (or before) t. But in order to determine

whether one defender raises the chance of the other’s

dying at t, we need instead to compare the latter’s

chance of dying in the case world with his chance of

dying had the former not activated his device at t.

43

Thus, the relevant comparison for determining the

magnitude of a defender’s threat is that between the

case world and the nearest possible world in which he

doesn’t activate his device at (or before) t. This

comparison gives us the result that the defenders do

threaten each other at t. Specifically, it entails that

each defender’s act at t raises the chance of the

other’s dying from 0% to 50%.15

15 [redacted] offers the following as a counterexample to

this claim.

There is a trolley headed towards a switch with a 50% chance

of turning towards A and a 50% chance of turning towards B.

If A but not B presses a button the trolley diverts to B. If

B but not A presses a button the trolley diverts to A. If

both press a button simultaneously there is a 50% chance

that A’s device will cause the trolley to divert to B and a

50% chance that B’s device will cause the trolley to divert

to A. However, if both press a button simultaneously, A’s

child and B’s child will both be saved. In order to save

both of their children, A and B simultaneously press a

button.

44

3.4 The ‘prior threat’ principle

The next proposal we will consider appeals to what I will call

The Prior Threat Principle (PTP): If person A threatens

person B, then the act by which A threatens B is

permissible only if B poses an impermissible

threat to A, and A’s threat is initiated after B’s

threat is initiated.

PTP entails that symmetrical self-defenders’ acts are

impermissible. As we saw, this leads to contradiction

[redacted claims that there is no reason against A and B

simultaneously pressing the button, and that therefore even though

it is true that if both simultaneously press a button and the

trolley is diverted to B, A causes the threat that B now faces,

this fact has no moral significance; it does not amount to ‘posing

a threat’ in the morally significant sense.

I think this is mistaken. Even if A and B would be justified

in pressing their buttons at the same time in order to save their

children, it is still morally significant that had A not pressed

the button when B did, B’s chance of death would’ve been 0% rather

than 50%, and vice versa.

45

in conjunction with each of the key claims that we want

to preserve. However, we can get around this problem by

incorporating in PTP a clause of the Standard View’s

Defensive Condition. On this proposal, one has a liability-

based permission to threaten to kill a liable attacker

in self- or other-defense if and only if the act by

which one threatens the attacker is necessary,

proportionate, and is performed only after the liable

attacker’s threat is initiated. Like the proposal

considered earlier of appealing to the relevance of

intention to permissibility, the appeal to PTP relies

on the idea that although each defender is liable to

defensive killing, neither has a liability-based

permission to kill the other. In this case, because

each defender violates PTP, neither satisfies Defensive

Condition.

One problem with PTP is that it seems to rule out

the permissibility of preventive defensive killing. In

at least some cases, it’s hard to deny that one is

46

permitted to kill a person when she doesn’t yet pose a

threat to one. For example, consider

Symmetry Case 5. Black has knowledge that Blue has

acquired a small missile, which Blue plans to

launch at Black at high noon. Black is almost

certain that Blue will launch the missile.

Moreover, he knows that the only way he can

prevent Blue’s missile from killing him is by

launching his own missile to intercept Blue’s.

However, Black has only a very small window of

opportunity in which to fire his missile. Unless

he launches his missile before Blue launches his,

Black will miss his window, and will then be

powerless to thwart Blue’s attack. Black is also

reasonably certain that if he launches his missile

now, it will reach Blue’s location moments after

high noon; thus, if Blue launches his missile at

noon, it will be intercepted by Black’s missile

almost immediately, and the debris from the

47

resulting explosion will kill Blue. Although Black

intends only to block Blue’s missile, he is aware

that his act of preventive self-defense would

probably kill Blue as a side effect. Black

launches the missile. As expected, it intercepts

Blue’s missile right near Blue’s location. Blue is

killed by debris from the explosion.

In Symmetry Case 5, Black appears to have an obvious

justification for killing Blue. Moreover, Blue’s death

is expected to be, and is ultimately, an unintended

side-effect of Black’s taking the necessary means of

defending his life. Finally, Black’s act causes Blue’s

death only after Blue culpably initiates his threat.

Thus, one cannot argue that Black’s act is

impermissible on the grounds that Blue still had time

to change his mind about attacking Blue, and so wasn’t

yet culpable for attempting to kill Blue. But because

Black’s threat is initiated before Blue’s, PTP entails

that Black’s act is impermissible. According to PTP, in

48

this case, Black must allow Blue to kill him unjustly.

This seems morally perverse.

PTP also seems to yield implausible results in

Symmetry Cases 3 and 4. Recall that in these cases, each

defender is threatened by a runaway trolley, and there

is no available procedure such as a fair coin toss to

determine the just distribution of harm. Because the

defenders activate their devices simultaneously, each

attempting to force the trolley onto the other’s track,

PTP entails that they act impermissibly. This seems

somewhat counterintuitive; but even if it were correct,

its correctness would seem to have nothing to do with

PTP. To explain the alleged impermissibility of each

defender’s act, we might appeal to the distinction

between killing and letting die, or to the distinction

between risking killing and letting die, or to the claim

that each defender activated his device while knowing

that he might deprive the other of an equal chance of

survival. But the claim that neither defender attacked

49

first seems insignificant, at least in this particular

case.

PTP seems both intuitively implausible and

explanatorily inert. We should look elsewhere for a

solution to the problem of symmetrical threats.

3.5 Appealing to the Distinction between Killing and Letting Die

One proposal we haven’t considered is that

symmetrical self-defense constitutes a special

exception to the Standard View.16 One could argue that the

upshot of the impossibility result proved in section 2

isn’t that the Standard View is false, but that it

simply fails to apply to cases of symmetrical self-

defense. One possible explanation for this is that the

Standard View is grounded in considerations of justice,

and that justice requires an imbalance of morally

relevant features between two or more agents that is

absent in cases of symmetrical self-defense. If this is

right, then we should appeal to other morally relevant

16 [redacted]proposed this in conversation.

50

considerations to determine whether each defender acts

permissibly.

One such consideration is the moral significance

of the distinction between killing and letting die. If

we suppose that killing is morally worse than letting

die, then since each defender might kill if he shoots,

but would merely let die if he allowed the other

defender to kill him, it seems each ought to refrain

from threatening to kill the other.

Of course, we shouldn’t think that killing is

always morally preferable to letting die. For example,

suppose that in

Symmetry Case 6. White culpably threatens to kill

the innocent Grey. Red can stop White’s threat

easily, and with no risk to himself, by shooting

and killing White. Instead, Red stands by and

allows White to kill Grey.

Clearly Red’s allowing Grey to die isn’t morally

preferable to Red’s killing White. In general, letting

51

an innocent person die isn’t morally preferable to

killing an agent who unjustly and culpably threatens to

kill her. But now consider

Symmetry Case 7. Indigo and Orange aim their rifles

at each other, each intending to kill the other.

Just before he would have pulled the trigger,

Indigo has a sudden and genuine change of heart.

Having decided that killing Orange would be wrong,

he lowers his rifle, thereby allowing Orange to

shoot and kill him.

According to the current proposal, Indigo’s allowing

himself to be killed is morally preferable to his using

deadly force to defend himself against Orange’s

culpable threat. But now suppose that Indigo’s deciding

not to shoot makes him innocent (i.e. non-liable). As

Symmetry Case 6 illustrates, allowing an innocent person

to die isn’t morally preferable to killing the culpable

agent who unjustly threatens to kill her. In Symmetry

Case 7, since indigo is innocent and Orange culpably

52

threatens to kill Indigo, it seems Indigo’s threatening

to kill Orange would have been morally preferable to

his allowing Orange kill him.

One might reject this claim, citing the fact that

if Indigo had threatened to kill the culpable Orange

then, assuming the case would have been one of

symmetrical self-defense, Indigo would then have had

the same morally reprehensible configuration of mental

states as Orange, and so would have been just as

culpable as Orange. Indigo’s threatening to kill Orange

would not have been morally preferable since, in that

case, Indigo would not have been innocent. But this

response depends on the assumption that one’s (perhaps

non-culpably) allowing a culpable threatener to kill an

innocent person is morally preferable to one’s culpably

threatening to kill that threatener. This seems

dubious.

One might argue that from each defender’s

perspective, attacking raises the chance of both

defenders dying, which would make the outcome worse

53

than if one or the other had refrained from attacking.

(Again, the possibility of both defenders dying is

compatible with its being the case that each will kill

the other if the other doesn’t kill him.) However as we

have seen there are cases of symmetrical self-defense,

such as Symmetry Case 3, in which one and only one defender

will die and each is equally responsible for this

inevitable outcome. In such cases, the outcome won’t be

better if one defender refrains from threatening the

other, and so the appeal to lesser evil won’t work.

For the foregoing reasons, it seems doubtful that

appealing to the killing/letting die distinction will

give us a solution to the problem of symmetrical

threats. Moreover, even if I am wrong about the moral

relevance of this distinction in cases of symmetrical

self-defense, it isn’t clear that the motivation I

cited for appealing to this distinction in the first

place, namely that considerations of justice don’t

apply in cases of symmetrical self-defense, is sound.

Why should justice be silent due to the symmetrical

54

conditions of the defenders? Why shouldn’t we say, in

accordance with the Just Distribution View, that justice

requires a coin flip (or some reasonably close

approximation) in order to determine how harm should be

distributed between the two defenders? Given the

availability of this alternative, to claim that justice

is silent in cases of symmetrical self-defense seems

objectionably ad hoc.

3.6 Appealing to Agent-relative permissions: Quong’s Principle of

Defensive Killing

In this section, I will consider a challenge to

assumption (c)—the claim that neither one of two

symmetrical self-defenders has a non-liability-based

permission to threaten the other. Specifically, I will

consider whether an appeal to agent-relative permissions can

avoid the symmetrical dependence of the deontic status

of one defender’s act on the deontic status of the

other’s act. Perhaps the most developed theory of self-

defense that appeals to agent-relative permissions has

55

been defended by Jonathan Quong, who proposes the

following principle:

Principle of Defensive Killing (PDK):

x can permissibly kill y in self-defense provided

that (i) y will otherwise kill x, (ii) killing y

is the only reasonable way of saving x’s life, and

(iii) x has not waived or forfeited his permission

to kill in self-defense.17

PDK implies that in at least some cases, it is

permissible for one to kill a non-liable person in

self-defense. For example, consider the case in which

the large man will crush you to death unless you

vaporize him. PDK implies that you may kill him if this

is the only reasonable way of saving your life, and

17 See Quong, ”Killing in Self-Defense,” p. 518.

Specifically, PDK states that you can permissibly kill person X in

self-defense provided (i) Killing X is the only reasonable course

of action that can save your life, and (ii) You have not waived or

forfeited your permission to act in self-defense.

56

provided that you haven’t done anything to forfeit your

permission to kill in self-defense. As Quong points

out, PDK also entails that the large man is permitted

to try to preempt your defensive threat by killing you

before you can kill him, as long as he also satisfies

the conditions of PDK. Supposing he satisfies these

conditions, if you try to vaporize him with your laser

gun, he is permitted to pull out his own laser gun and

try to vaporize you first. On Quong’s view, this is a

case of what we might call morally symmetrical self-

defense. In such cases, neither defender is liable to

defensive killing, and each retains his permission to

kill the other in self-defense.

One might think that PDK could plausibly be

extended to cases of symmetrical self-defense. This

thought may seem initially tempting because it may be

natural to suppose that if we have agent-relative

permissions to kill in self-defense, then when the

agent-neutral factors governing the permissibility of

symmetrical self-defenders’ acts are in perfect balance

57

their agent-relative permissions should apply and

generate the desired moral symmetry between them.

However, this depends on the assumption that in the

course of defending themselves neither defender does

anything to forfeit his agent-relative permission to

kill in self-defense. According to Quong, it is

sufficient for forfeiting one’s agent-relative

permission that one forfeit one’s right not to be

killed i.e. that become liable to be killed.18 This is

very plausible. However, it seems to imply that the

question of whether each defender is liable to

defensive killing is metaphysically and epistemically

prior to the question of whether each satisfies PDK

and, thus, whether each retains his agent-relative

permission to kill the other. To determine whether

symmetrical self-defenders retain their agent-relative

permissions, we must first determine whether they are

liable. However, if each defender meets the subjective

criteria necessary for liability, then we will

18 Ibid., pp. 519-520

58

encounter the same problem as before. In order to say

that one such defender is liable, we need to determine

whether the other defender is liable. But in order to

determine this, we need to know whether the other

defender threatens a liable person. Thus, the appeal to

agent-relative permissions just pushes the problem

around.19

Indeed, the problem will reemerge even if we adopt

Quong’s own account of liability to defensive harm,

which he calls the moral status account (MSA). On this

account, one becomes liable to defensive harm when one

meets certain minimal conditions of moral 19 Again, this assumes that each defender meets the relevant

subjective criteria of liability. The problem does not merely get

pushed around if the symmetrical self-defenders are both non-

responsible threateners, as for example in Otsuka’s case in which they

are merely falling towards each other. In that case, on Quong’s

view, neither is liable to defensive harm, and both have an agent-

relative permission to kill the other in self-defense. However,

here I am focusing on cases in which each defender is morally

responsible for the act by which he threatens the other.

59

responsibility, and from an evidence-relative

perspective one treats a person with less than the

amount of respect and consideration that she is in fact

due. This happens when (i) the evidence-relative

permissibility of one’s act depends either on the

assumption that those who are harmed (or might

foreseeably be harmed) by one’s acting in this way are

liable to the harm one threatens to inflict on them, or

else on false moral beliefs, and (ii) those one

threatens to harm aren’t liable to such harm i.e. they

are innocent.20 Quong elucidates his view in the

following passage.

Liability to defensive harm should depend on

whether, in imposing a threat of harm on others,

we treat those others as having fewer rights, or

less stringent rights, than people normally

possess. When we act in this way, we go out on a

20 This formulation of Quong’s view is slightly different

from his own formulation, although the substantive content is the

same. See Quong (2012), pp. 67-68.

60

moral limb: we knowingly take the risk of treating

others as if they lack important moral rights, and

we also know (or ought to know) that the

permissibility of our actions depends on making

this judgment about the moral status of others.

When we take such a risk, it is only fair that we—

as opposed to the innocent person who now faces a

threat of harm—may be liable to defensive harm

when our judgment about the moral status of others turns out to

be mistaken … When, and only when, our actions have

this particular feature—treating others with

something less than the concern and respect they

are due—do we make ourselves liable to defensive

harm.21

Because one is liable to defensive harm when and only

when one treats others with less concern and respect

than one owes them, one doesn’t become liable by

threatening to harm a person who is in fact liable to

such harm. Thus, if I meet the minimum requirements of 21 Quong (2012) p. 72, italics added

61

moral responsibility, and threaten you in a way that,

from an evidence-relative standpoint, treats you with

less concern and respect than a non-liable person is

due, then whether I am liable to defensive harm depends

on whether you are due such concern and respect; if you

are liable to the harm that I threaten to cause you,

then I do not treat you with anything less than the

concern and respect that you are owed. Thus, all else

equal, I am not liable to defensive harm; I retain my

right not to be killed and, assuming I satisfy the

other two conditions of PDK, I retain my agent-relative

permission to kill in self-defense.

Now suppose for reductio that (i) Smith and Jones

are symmetrical self-defenders who meet the minimal

requirements of moral responsibility, (ii) the

evidence-relative permissibility of the act by which

Smith threatens to kill Jones depends on the assumption

that Jones is liable to defensive killing, and (iii)

Jones is liable to defensive killing. Then Smith

doesn’t treat Jones with anything less than the concern

62

and respect he is in fact due. For, in this case, Jones

is due no more or less concern and respect than what is

owed one who is liable to be killed. Thus, according to

MSA, Smith isn’t liable to defensive killing for the

act by which he threatens Jones. But this violates

Symmetry, which requires that both defenders have the

same liability-status. Next, suppose that (i) and (ii)

hold, but Jones isn’t liable to defensive killing. Then

he is innocent in the relevant sense. Thus, in

threatening to kill Jones Smith fails to show Jones the

concern and respect that Jones is in fact due.

According to MSA, Smith is therefore liable to

defensive killing. Again, this violates Symmetry.

Given Quong’s plausible restriction on agent-

relative permissions to kill in self-defense, in cases

of symmetrical self-defense we cannot determine whether

one defender retains his permission to kill without

first determining whether he is liable to defensive

killing, which we cannot determine unless we first

determine whether the other defender is innocent, which

63

we cannot determine without first determining whether

the former defender is innocent. Thus, it seems, our

story about how one retains or forfeits one’s agent-

relative permission in cases of symmetrical self-

defense is going to be circular.

(My main aim here isn’t to criticize MSA but to

demonstrate how the problem of symmetrical threats

reemerges when we ask whether each defender has done

something to forfeit his agent-relative permission to

kill in self-defense. MSA is one of many different

accounts of liability on which the problem can arise if

we appeal to agent-relative permissions.)

One way around the problem would be to claim that

symmetrical self-defenders retain their agent-relative

permissions to kill in self-defense even after they

have become liable to defensive killing. The following

case might be thought to support this view.

Symmetry Case 8. Smith is on his way to shoot

Jones. Because he is threatening to violate

another’s right not to be killed, Smith is liable 64

to attack. While en route, Smith is ambushed by

another attacker, Moe, who threatens to kill

Smith. Smith shoots and kills Moe before Moe is

able to kill Smith.22

Suppose we say that Smith’s use of deadly force in

self-defense here is permissible, notwithstanding that

Smith is liable to defensive killing. Then perhaps

Smith’s agent-relative permission to kill in self-

defense can remain intact despite his being liable to

such killing. One difference between Symmetry Case 8 and

a case of symmetrical self-defense is that the former

involves three agents rather than two; however, it’s

not clear whether this should matter. Suppose that

while en route to kill Jones, Smith is ambushed by Jones

instead of Moe. This might seem relevantly similar to a

case of symmetrical self-defense. We might therefore

maintain, again, that Smith can permissibly threaten to

kill Jones, notwithstanding that Smith is liable to

22 This case is adapted from one suggested to me by

[redacted].

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defensive killing. And, if this is true, then Jones

plausibly has a symmetrical permission to threaten to

kill Smith.23

But what grounds such permissions? One

possibility is that each defender implicitly consents

to being attacked when he goes snooping around with his

sniper rifle However, as McMahan points out, neither

actual nor hypothetical consent can plausibly justify

the killing of another person in the kinds of cases

we’re considering.24 The proposal that each defender

retains his permission to kill in such cases also has

disturbing implications for the ethics of war, assuming

that the moral principles that govern combat in war are

the same or are at least similar to those that govern

individual self-defense outside of war. On this

assumption, the current proposal seems to imply that

23 I am grateful to [redacted] for emphasizing this point in

correspondence.

24 McMahan argues persuasively for this point in his “On the

Moral Equality of Combatants,” The Journal of Political Philosophy 14

(2006): 377-393.

66

enemy combatants can permissibly engage in war as long

as no innocent bystanders are threatened. This seems

highly implausible, despite the fact that bystanders

are almost always threatened during war.

I tentatively conclude that the appeal to agent-

relative permissions cannot plausibly resolve the

puzzles posed by cases of symmetrical self-defense. And

since it is hard to see what other non-liability-based

permissions symmetrical self-defenders could have for

threatening to kill each other, it is hard to see how

we can plausibly resolve the inconsistency that has

been the central focus of this paper by rejecting

assumption (c) i.e. the assumption that neither

symmetrical self-defender has a non-liability-based

permission to threaten the other.

4. Conclusion

The Standard View is simple and powerful. It accounts for

many people’s intuitions about the permissibility of

defensive killing. However, it stumbles on the problem

of symmetrical threats; it assumes that the

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permissibility of defensive killing depends on the

impermissibility of a liable person’s act, but in

symmetrical self-defense there is no such asymmetry. I

have considered several ways of resolving this problem,

including limiting the application of the Standard View to

cases other than those involving symmetrical self-

defense. However, none of the proposals that I have

considered seems plausible. Perhaps one or more of

these proposals, or some modified versions of them, can

be successfully defended. Otherwise, it seems,

liability to defensive killing and the permissibility

of defensive killing must be decoupled. I will conclude

the paper by briefly suggesting several different ways

this decoupling might happen. These suggestions are

highly inchoate and each is potentially problematic.

First, we could claim that one’s being liable to

defensive killing doesn’t ground a permission to kill

in self- or other-defense, but that it is impermissible

to threaten to kill even those who are liable to be

killed. Second, we could claim that one’s being liable

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to defensive killing doesn’t presuppose that one acts

impermissibly in either the fact-relative or the

evidence-relative sense. (In the previous section we

considered a view of this kind that appealed to the

claim that one retains one’s agent-relative permission

to kill in self-defense even after one has forfeited

one’s right not to be killed.) Third, we could claim

that the fundamental features of acts—including acts of

defensive killing—that make them permissible or

impermissible have essentially nothing to do with

liability. Obviously, those who endorse this view must

say what the alternative form of justification would

be. Exploring any of these proposals at greater length

is a project that is beyond the scope of this paper.

Appendix

We are assuming that if ERA is true then it either is,

or follows from, the most general true moral

69

principles. Granted this assumption, Evidential Symmetry,

DM, (a’), (b’), and ERA are jointly incompatible.

Suppose for reductio: 1. Smith’s evidence indicates that Smith’s act is

fact-relative impermissible and that he meets the subjective criteria necessary for liability.

Then,2. Smith’s evidence indicates

2’. Jones’s evidence indicates that Jones’s act isfact-relative impermissible and that he (Jones) meets the subjective criteria necessary for liability. (from 1 and Evidential Symmetry)

3. 2’ and ERA’s Evidence-Relative Condition jointly entail that Jones is liable to defensive killing.

4. ERA’s Evidence-Relative Condition is one of the true general moral principles or at least it can be derived from the set of true general moral principles. (assumption)

Therefore, 5. Smith’s evidence indicates:

5’. Jones is liable to defensive killing. (from 2,3, 4, and the definition of “S’s evidence indicates that p”)

6. 5’, (a’), and ERA’s Defensive Condition jointly entailthat the act by which Smith threatens to kill Jones is fact-relative permissible.

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7. ERA’s Defensive Condition is one of the true general moral principles (or at least it can be derived from the set of true general moral principles)

Therefore,

8. Smith’s evidence indicates that the act by which he threatens Jones is fact-relative permissible. In other words, Smith’s act is evidence-relative permissible. (from 5, 6, 7, and the definition of “one’s evidence indicates that p”)

Therefore,

9. Smith’s act isn’t evidence-relative impermissible.In other words, it is not the case that Smith’s evidence indicates that Smith’s act is fact-relative impermissible. (from 8 and DM) Contradiction!

Next suppose:

10. Smith’s act is evidence-relative permissible .In other words, Smith’s evidence indicates that his act is fact-relative permissible.

Then,

11. Smith’s act isn’t evidence-relative impermissible. (from 10 and DM)

Therefore,

12. Smith’s evidence indicates:

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12’. Jones’s act isn’t evidence-relative impermissible. In other words, it is not the case that Jones’s evidence indicates that his (Jones’s)act is fact-relative impermissible. (by 11, and Evidential Symmetry Modus Ponens)

13. 12’ and Evidence-Relative Condition jointly entail that Jones isn’t liable to defensive killing.

Therefore,

14. Smith’s evidence indicates:

14’. Jones isn’t liable to defensive killing. (from 4, 12, 13, and the definition of “one’s evidence indicates that p”)

15. 14’, (a’), and Defensive Condition jointly entail that Smith’s act isn’t fact-relative permissible.

Therefore,

16. Smith’s evidence indicates that his act isn’tfact-relative permissible. In other words, Smith’sact isn’t evidence-relative permissible. (from 7, 14, 15, and the definition of “S’s evidence indicates that p”) Contradiction!

Since the premise that Smith’s act is evidence-relative

permissible and the premise that his act is evidence-

relative impermissible both lead to contradiction,

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Smith’s act is neither evidence-relative permissible

nor evidence-relative impermissible. But this

contradicts (b’), which states that each defender’s act

is either evidence-relative permissible or evidence-

relative impermissible. This completes the proof.

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