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If somebody is trying to kill you, it seems acceptable to act in self-defence, even if it necessarily means that you have to kill the other person in the act of doing so.

However, if you know someone wishes to kill you, but has not been given an opportunity to do so just yet, would it be unethical if you found and killed that person first yourself?

Of course we can never truly know such things, there's always a probability that we are wrong and this other person would've never attempted to kill us if left alone ... but surely our answer kind of depends on that probability? If the probability of them killing us if left alone is > 99 %, would we then be ethically justified in killing them first? What if it was > 99.9999% Or if it was = 50 %? Where do we draw the line, if a line even exists?

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    There's a time factor involved. If someone pulls out a knife and lunges at you, and you shoot him dead one second later, most people would consider it justified self-defense. In the U.S., you probably wouldn't be prosecuted. But if someone SAYS he's going to stab you some day, and you kill him before he can kill you, the morality is murkier. Some people will consider you immoral/amoral, while others will consider you a hero. But the law probably won't be on your side. – David Blomstrom Oct 2 '17 at 0:51
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    One of the key words is state. Most people regard the state as a virtual deity, and taking the law into your own hands is frowned on. But if the state cannot or will not protect you from a murderer, or if the murderer is actually sanctioned by the state, then you may have to consider "wild justice." It's biology (i.e. self-preservation) versus the law. But if you're caught, you will likely still be prosecuted, and your friends and neighbors may or may not condemn you, depending on the circumstances. I'd be interested in knowing what various notable philosophers had to say about this. – David Blomstrom Oct 2 '17 at 0:51
  • Some of this question has strayed into the very murky territory of thought experiment. For many philosophers these prove interesting, but they are flawed for physicalists, or perhaps monists in general. Consider that we do not live in a world where you can ever be any known % sure of someone's intent, our concepts of right and wrong have developed in such a world. If you postulate a world where one could know such a thing morality would be a different thing entirely, in fact the whole of society would be very different should such things be knowable. – Isaacson Oct 3 '17 at 6:38
  • You might want to work backward from the question of whether wars of defense are legitimate. If you cannot kill someone who actively wants to kill you, you are going to have a very hard time securing property rights. – user9166 Nov 20 '17 at 17:53
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Any reflection on the moral character of an action would have to take the environment into consideration, particularly the dominant norms. This principle should also remain valid if one does not regard them as absolutes, or even in the case one disagreed with their legitimacy (freedom of thought).

Hence you might want to divide the question into two distinct situations according to the environment: the state of nature and the state of society (a concept that traces back at least to Hobbes, but see also the Social Contract by Rousseau). This dichotomy might be useful.

In the state of nature1 where there is no police and no justice (and supposing that could really exist, since some state could still claim jurisdiction on the location) the decision would be in principle strictly personal, by definition; in other words down to the individual's own sense of ethics (or more likely, and at a lower level, survival instinct). This would, consciously or inconsciously involve weighing chances of survival when selecting courses of action; or in the extreme it would boil down to killing with one the various techniques animals use in nature (pursuit, ambush, face-to-face duel, poisoning, stealing food, etc.).

In the state of society the functions of police and judicial have been delegated to the state. According to the social contract, it is not up to an individual to make that type of adjudication. So a citizen who reasons like this would be in breach of the social contract and would undermine the rule of law.

In a state of society, this "thin line" you are contemplating is the laws of the land, specifically the criminal code, which explicitly forbids this kind of behaviour. On top of this, this runs against the dominant moral codes of most human societies, which consider killing bad per se.

Indeed, acting by pre-emptively killing someone would qualify as assassination, which carries among the highest penalties in criminal codes. Since those law codes are consciously written by people who carefully ponder a "scale of represession" (or whatever it would be called), this is an indication of the extent to which such a behaviour is considered a dangerous transgression for a society.

Going back to a similar situation in a state of nature, let us remember that an individual would also probably want to go back to "civilization" which (still and regardless of the specific conditions) enforces its dominant set of values. If the behaviour of the individual had violated those rules, the individual would also have to live later with it privately, or else need to justify themselves to their fellows; including (because of the practice of territorial claims, or even extraterritoriality) in front of courts.

And then the "thin line" would be back to the only admitted exception, i.e. self-defense. Perhaps with some allowances due to mitigating circumstances (due to the harshness of the conditions), as well as the presumption of innocence.


  1. "A primitive state of existence, untouched and uninfluenced by civilization or social constraints: when people lived in a state of nature." American Heritage
  • Good answer. But what if the state is corrupt or dysfunctional and won't protect you? What if the person who's trying to kill you is an agent of the state? A familiar example is the rogue cops that brutalize so many people. – David Blomstrom Nov 20 '17 at 17:52
  • That would be a rough proposition... As long as the state is still technically in place, it still has the potestas (I believe you might find such discussions in Grotius), so if one went against the laws, that would actually backfire by legitimizing the corrupt agents of that state. In that situation, the person would be branded a rebel and would then have double reason to fear for their own safety. Should they look at exile? – fralau Nov 20 '17 at 20:02
  • So a person has to choose between being branded a rebel vs branded a political dissident, a homeless bum, etc. Exile is a possibility, but another possibility is revolution. – David Blomstrom Nov 21 '17 at 22:20
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'However, if you know someone wishes to kill you, but has not been given an opportunity to do so just yet, would it be unethical if you found and killed that person first yourself?'

I think all that follows is that one is justified in preventing or in trying to prevent the other person from killing you. Whether in order to achieve this purpose it is necessary to kill the other person is a specific question not settlable apart from a context - not indicated in the question. Good question, though.

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