Writing later, St Thomas saw the need of qualifying this argument, which, taken absolutely, would make short shrift of lunatics and troublesome invalids generally, and would consecrate the principle of lynch-law. He puts in therefore these two qualifications
(1) "Man by sinning withdraws from the order of reason, and thereby falls from human dignity, so far as that consists in man being naturally free and existent for his own sake; and falls in a manner into the state of servitude proper to beasts. . . . And therefore, though to kill a man, while he abides in his native dignity, be a thing of itself evil, yet to kill a man who is a sinner may be good, as to kill a beast."
(2) "A beast is naturally distinguishable from a man: hence on this point there is no need of judgement. . . . But a sinner is not naturally distinguishable from just men; and therefore he needs a public judgement to make him out, and determine whether he ought to be slain for the benefit of the common weal."
The student should read the whole of Sum. Theol. 2a-2ae, q. 64, art. 2 and 3 (Aquinas Ethicus, II, pp. 4o-42), whence these extracts are taken.
Of God and His Creatures: 3.147