Let us pause to take stock of what we have so far achieved. We began in Chapter 1 with some very general considerations about the nature of rights from which we were able to identify the structure of defensive rights as justifications consisting in liberties to perform acts which would otherwise be prohibited. We have begun to develop a general model of defensive rights which traces their normative grounds to rights to things as well as general and specific duties of care. We have introduced and explained the intrinsic limitations of necessity, imminence, and proportionality and defended a specific interpretation of them. Do we now have a satisfactory account of self-defense?
Unfortunately we do not. What we have is the beginning of an understanding of the structure and operation of the right of self-defense, of when it arises, what its intrinsic limits are, as well as its relation to other normative features. We have a partial map of the moral topology of self-defense. But an extremely important and difficult question remains unanswered.
That question concerns what happens when a defensive right rubs up against someone else's positive right. What happens if necessary and proportionate defensive action involves harming another party in such a way that the harm would in ordinary circumstances be a violation of that person's rights? What can justify a putative defender in acting in such circumstances? These questions take us beyond the explanatory power of the considerations that we have so far introduced. We have said that defensive rights arise out of rights to things and duties of care, but the justification provided by these relationships have definite limits. They can explain why it is the defender's business to undertake defensive measures; why he has a legitimate interest in doing so, but they don't explain what makes the aggressor, as it were, morally vulnerable to his harmful actions. Similarly the notions of necessity, imminence, and proportionality explain why the infliction of certain harms is not impermissible given a particular balancing of harms and goods. But what is missing is an explanation of why we are justified in inflicting the harm on a particular person—a person whom we can assume has interests and rights of his or her own.