Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

Aristotle defines an expression as follows: “An expression is a vocal sound significant by convention, whose parts, when separated, signify something as a word, not as affirmation or negation.” So Aristotle in On Interpretation I¹ and St. Thomas in the same place, lecture 6².

Here expression is defined in general, with respect both to perfect and imperfect expression, whether simple or hypothetical. The first three clauses have been explained in the definition of the term. The clause “whose parts signify when separated” can be explained no better than by saying that an expression is subordinated to a composite concept, as is drawn from St. Thomas, On Interpretation I, lecture 6³, so that within the expression itself there are parts that compose it and correspond to distinct, separate concepts as components of one whole. In virtue of the first point, expressions of a compound figure, such as respublica, signifer, and so on, are not expressions, because they do not correspond to a double concept but to one, even though the vocal sounds separated from the expression signify different things. In virtue of the second point, an expression is distinguished from a complex term, because a complex term also has significant parts and several concepts, but in the manner of a part that further composes, not in the manner of a composite whole.

Aristotle and St. Thomas include the final clause to indicate that the parts of an expression must at least be significant “as a word,” that is, as a term; they need not be affirmation and negation. For even if an expression is hypothetical and has parts consisting of affirmation and negation, nevertheless simple affirmation and negation themselves are also expressions and must be resolved into significant parts, which signify only as a simple term and not as affirmation. Therefore, in defining expression in general, what is common to every expression is included: namely, that it has parts that are simple terms or expressions.

Expression is first divided into perfect and imperfect. According to the Divine Thomas, On Interpretation I, lecture 7⁴, a perfect expression is one “that produces a perfect sense in the mind of the hearer”; an imperfect expression is one “that produces an imperfect sense.”

A sense is called perfect or imperfect not because one involves assent and the other does not, or one involves truth or falsity and the other does not, but precisely because the perfect expression does not leave the intellect suspended, as though awaiting the whole sense of the expression; rather, it completes and expresses the entire thought perfectly, as when I say, “God is the highest good.” An imperfect expression, by contrast, does not explain the entire sense, but seems to suspend the intellect, as when I say “Peter disputing,” “if you should sleep,” “when he was passing by,” and the like. Such expressions are very close to complex terms and differ only in their mode of composing and joining terms: in an expression, they are joined in the manner of a whole, though an imperfect one that leaves the intellect suspended; in a complex term, they are joined in the manner of a composed part.