The order of treatment in the discussion which follows

We have considered both those who wage war and on what grounds war may be waged. It follows that [in the following discussion] we should determine what is permissible in war, also to what extent and in what ways it is permissible. What is permissible in war is viewed either absolutely or in relation to a previous promise. It is viewed absolutely, first, from the standpoint of the law of nature, and then from that of the [volitional] law of nations. Let us see, then, what is permissible by nature.

On things which are necessary to attain the end in view

First, as we have previously said on several occasions, in a moral question, things which lead to an end receive their intrinsic value from the end itself. In consequence, we are understood to have a right to those things which are necessary for the purpose of securing a right, when the necessity is understood not in terms of physical exactitude but in a moral sense. By right, I mean that which is strictly so called, denoting the power of acting in respect to society only. Hence, if otherwise I cannot save my life, I may use any degree of violence to ward off him who assails it, even if he should happen to be free from wrong, as we have pointed out elsewhere. The reason is that this right does not properly arise from another's wrong, but from the right which nature grants me on my own behalf.