Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

The principal species of perfect expression is enunciation or proposition, and it is treated extensively in Logic because it serves above all for the syllogism and reasoning. We take proposition and enunciation to be the same, since this usage has prevailed even among the learned, as is clear in ordinary disputations, in which we use the name proposition more often than enunciation. St. Thomas likewise uses the name proposition for enunciation in Summa Theologiae I, question 13, article 12. Elsewhere, however, with Aristotle, he distinguished the function of a proposition from the simple function of an enunciation, in Posterior Analytics I, lecture 5¹ and Opusculum 48, treatise On the Syllogism Simply, chapter 1²: namely, a proposition adds to an enunciation that it be put forward in order to infer something in argumentation.

An enunciation, then, is defined as “an expression signifying the true or the false by asserting,” as when I say, “Man is an animal.” So Aristotle, On Interpretation I³, and St. Thomas, lecture 7⁴. “Expression” is put in place of the genus; “signifying the true or the false” in place of the difference, by which enunciation is distinguished from other expressions, whether imperfect or perfect but not expressing truth. The clause “by asserting” is included not because it is formally found in every proposition, but to indicate how the verb in a simple proposition serves to signify the true or the false, namely, by way of assertion. This is commonly done by a verb in the indicative mood, or must be reduced to it. In a simple enunciation, truth is signified assertively, whether by affirming or denying. In hypothetical propositions, although one does not always proceed assertively, as in conditionals in a non-indicative mood—for example, “If you were to strike Peter, he would kill you”—nevertheless, through some understood sense, the truth of the propositions can be reduced to assertion in the same way that it can be reduced to categorical propositions.

To make clear what signifying the true is, note from St. Thomas, On Interpretation I, lecture 7⁵, that truth is in an enunciation as in a sign of an intellect’s apprehending the true or the false; but the true or the false is in the mind (that is, in the mind’s judgment) as in a subject, as is said in Metaphysics VI⁶; in the object, however, it is as in a cause, because, as is said in the book of the Categories, “from the fact that a thing is or is not, an expression is true or false.”⁷ Here St. Thomas is speaking of a vocal or non-judicative enunciation when he says that truth is in an enunciation as in a sign of an intellect’s apprehending the true or the false; for when speaking of a mental, judicative enunciation, truth is in it as in a subject.

Nevertheless, since a mental, judicative enunciation is also significant and representative of objective truth, we further say that signifying the true or the false in a mental proposition is nothing other than signifying in the manner of composing or dividing, so that something is signified as judgeable. When a predicate is joined to or separated from a subject, the result is that it conforms to and is adequate to the thing itself, or fails to conform to it—not only by conformity or nonconformity of likeness, but by attribution and conjunction of the predicate with the subject. For likeness to an external thing is also found in simple apprehension. Thus St. Thomas teaches in Summa Theologiae I, question 16, article 2, that simple apprehension or sense is true but does not know the true, because it does not know the conformity of truth: it does not apply something to something else or remove it from it. Therefore, to signify the application of one thing to another—or rather, to signify a thing by applying one thing to another in the manner of what is judgeable—is to signify the true or the false. The truth of a proposition itself is not the very signification of the true or the false, but the conformity or adequation to the thing signified that results from such signification. Those who said in a single phrase that to signify the true is to signify that a thing is as it is, and to signify the false is to signify it otherwise than it is, intended to express all of this. But this is not merely signifying in some manner, as a syncategorematic term does, nor merely signifying something simple, as a categorematic term does; it is signifying something composite, or an application of one thing to another, which is an enunciative expression. We shall say below, question 5, how truth or falsity itself is not the essence of a proposition but rather its signification, from which truth or falsity results; and how one truth is formally neither greater nor less than another.

But note that some propositions falsify or destroy their own truth, just as others destroy their own possibility or necessity. This occurs when the very verification of the proposition entails that it is false—that is, from its being as it is signified by that proposition. For example, if I say “Every proposition is false,” or “This proposition is false,” while pointing to that very proposition: from its being as it signifies, it follows that it is false. Such propositions are simply false, because although they have verification, that verification is not simply truth, since falsity follows from it. For evil arises from any defect, and that from which falsity follows is simply not true.