Falsity, strictly speaking, cannot be found in things which depend upon God, and which necessarily are conformed to His Divine Mind, as products of art agree with the mind of the artist. The falsity of a thing, therefore, consists, properly speaking, in its divergence from the intellect on which it depends, and also accidentally (per accidens) it consists in its divergence from the mind on which it does not depend; wherefore falsity cannot exist in relation to the Divine Mind, except, perhaps, we may admit in voluntary agents alone, in whose power it is to withdraw themselves from the order of the Divine Plan; and in this consists the evil of sin, and hence sins are called falsities and lies. In relation to our human mind, to which natural things are related accidentally, they may be called false by accident or notionally, as that may be called false which is signified or represented in false speech or idea, or by way of cause, inasmuch as they may be sources of falsehood. Things which in external attributes appear like other things are called false in relation to those things, as if it were said that vinegar is honey and tin silver.
Nor are the senses false as regards their proper object unless by accident, and in cases when through indisposition of the organ the sensible form is not properly received - thus by defect of the tongue sweet may taste bitter. But there may be a false judgment even in senses rightly disposed as regards things beyond their natural sphere, or as regards something which is the object of more than one sense, accidentally, or in consequence of its being referred to something else.
Neither is there falsity in the essence of the intellect, for a natural thing does not fall short of its existence according to its form, though it may fail accidentally or in something that follows from it; and in like manner the faculty of knowledge does not fail in the knowledge of what informs it, but only accidentally, or as regards what ensues from it. Therefore the intellect is not deceived as regards the essence of a thing, though in the process of reasoning it may attribute to things whose nature it understands something which does not follow therefrom or is opposed thereto, for the intellect so judging conducts itself as sense does as regards things which are the object of more senses than one. True and False are contraries, for falsity is to apprehend that to be which is not, or that not to be which is; and as truth assigns the acceptation adequate to the thing, so falsehood assigns an acceptation which is not adequate to it, and determines the subject.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni