 As a private person one may rightfully use force in defense of oneself or others even if the force is such that one foresees it is likely or even certain to kill; but one’s intention in using such lethal force must not be to kill, but only to disable and block the attack (and less lethal force would not have met the need for defensive blocking of the assault). Aquinas’ discussion of this (ST II-II q. 64 a.7) is the locus classicus for what later became known, unhappily, as the “principle” of double effect, whose real core is the thought that moral principles bear differently on kinds of action specified by intention (e.g. to kill) from the way they bear on behavior chosen with foresight that it will (probably or even certainly) kill as a side-effect (praeter intentionem – outside the acting person’s intention).