 Aquinas’ thesis is that lying, properly defined, is always to be excluded as to some extent wrong. The thesis is often misunderstood as being premised on the thought that lying is contrary to the natural function of tongue or speech, a thought that has often been transposed into an effort to explain his theses about the wrong kinds of sex act. But, as has been seen, his sex ethics has another and more plausible basis, and so, it seems (albeit less clearly), has his ethics of lying. Though all his treatments include something like “words are naturally signs of what one understand” (ST II-II q. 110 a. 3) or “speech was invented for expressing the conceptions in one’s heart” (Sent. III d. 38 a. 3c), this statement is pointing to a more explanatory and prior premise, discernible though never adequately articulated so as to show its relation to basic human goods. This prior premise turns on his definition of lying as one’s assertion of what one believes to be false. Whenever one asserts, one affirms as true two propositions: explicitly the proposition one articulates as true despite one’s belief that it is not, and implicitly the proposition that one believes what one is assertively articulating. So Aquinas seems to locate the essential wrongfulness of lying in this intentional dissonance between the self presented or reported and one’s real self: a duplicitas. It is often reasonable and even morally necessary to hide one’s beliefs, and this, not “deception”, is what Aquinas means by the “prudent dissimulatio” he thinks justifiable in appropriate contexts. But one should not do so by the spurious self-projection entailed by asserting what one believes false. Pretended flight as a device for luring an unjust enemy into an ambush can be right in a just war, but lying to the enemy is wrong, although its gravity is much diminished by the duty not to reveal the truth to the enemy, a duty compatible with the coexisting duty not to lie. Subsequent scholars in his tradition have wondered whether the conditions of discourse with an unjust opponent do not, at least in many circumstances, defeat the presumption that a grammatically indicative statement asserts what it is put forward to seem to assert.