Thomas Aquinas

“ Therefore truth and falsehood exist in the intellect.
I answer that, Just as a thing has being by its proper form, so the knowing faculty has knowledge by the likeness of the thing known. Hence, as natural things cannot fall short of the being that belongs to them by their form, but may fall short of accidental or consequent qualities, even as a man may fail to possess two feet, but not fail to be a man; so the faculty of knowing cannot fail in knowledge of the thing with the likeness of which it is informed ”

“ But because falsity of the intellect is concerned essentially only with the composition of the intellect, falsity occurs also accidentally in that operation of the intellect whereby it knows the essence of a thing, in so far as composition of the intellect is mixed up in it. This can take place in two ways. In one way, by the intellect applying to one thing the definition proper to another; as that of a circle to a man. Wherefore the definition of one thing is false of another. ”

“ And since every thing is denominated simply by what belongs to it "per se," but is denominated relatively by what belongs to it accidentally; a thing indeed may be called false simply when compared with the intellect on which it depends, and to which it is compared "per se" but may be called false relatively as directed to another intellect, to which it is compared accidentally. Now natural things depend on the divine intellect, as artificial things on the human. Wherefore artificial things are said to be false simply and in themselves, in so far as they fall short of the form of the art ”

“ Falsity asserts something, for a thing is false, as the Philosopher says (Metaph. iv, 27) , inasmuch as something is said or seems to be something that it is not, or not to be what it really is. For as truth implies an adequate apprehension of a thing, so falsity implies the contrary. Hence it is clear that true and false are contraries.
Reply to Objection 1: What is in things is the truth of the thing; but what is apprehended, is the truth of the intellect, wherein truth primarily resides. ”

“ Because contraries, and opposites by way of privation, are by nature about one and the same thing, therefore there is nothing contrary to God, considered in Himself, either with respect to His goodness or His truth, for in His intellect there can be nothing false. But in our apprehension of Him contraries exist, for the false opinion concerning Him is contrary to the true. So idols are called lies, opposed to the divine truth, inasmuch as the false opinion concerning them is contrary to the true opinion of the divine unity. ”

“ The affection of sense is its sensation itself. Hence, from the fact that sense reports as it is affected, it follows that we are not deceived in the judgment by which we judge that we experience sensation. Since, however, sense is sometimes affected erroneously of that object, it follows that it sometimes reports erroneously of that object; and thus we are deceived by sense about the object, but not about the fact of sensation. ”

“ Sense, then, has no false knowledge about its proper objects, except accidentally and rarely, and then, because of the unsound organ it does not receive the sensible form rightly; just as other passive subjects because of their indisposition receive defectively the impressions of the agent. ”

“ In things that depend on God, falseness cannot be found, in so far as they are compared with the divine intellect; since whatever takes place in things proceeds from the ordinance of that intellect, unless perhaps in the case of voluntary agents only, who have it in their power to withdraw themselves from what is so ordained ”

“ I answer that, True and false are opposed as contraries, and not, as some have said, as affirmation and negation. In proof of which it must be considered that negation neither asserts anything nor determines any subject, and can therefore be said of being as of not-being, for instance not-seeing or not-sitting. ”

“ Hence, for instance, it happens that on account of an unhealthy tongue sweet seems bitter to a sick person. But as to common objects of sense, and accidental objects, even a rightly disposed sense may have a false judgment, because it is referred to them not directly, but accidentally, or as a consequence of being directed to other things. ”

“ Regarding this, Augustine says (Soliloq. ii, 6) : "We call those things false that appear to our apprehension like the true:" and the Philosopher says (Metaph. v, 34) : "Things are called false that are naturally apt to appear such as they are not, or what they are not." In this way a man is called false as delighting in false opinions or words, and not because he can invent them; for in this way many wise and learned persons might be called false, as stated in Metaph. ”