The short discourse on the expression of thought by language (περὶ Έρμηνείας, De Interpretatione) is based on the Platonic division of the sentence (λόγος) into noun and verb (ὄνομα and ῥῆμα.) Its point is to separate the enunciative sentence, or that in which there is truth or falsity, from other sentences; and then, dismissing the rest to rhetoric or poetry (where we should say grammar), to discuss the enunciative sentence (ἀποφαντικὸς λόγος), or enunciation (ἀποφανσίς), or what we should call the proposition (De Int. chap. 4). Here Aristotle, starting from the previous grammar of sentences in general, proceeded, for the first time in philosophical literature, to disengage the logic of the proposition, or that sentence which can alone be true or false, whereby it alone enters into reasoning. But in spite of this great logical achievement, he continued throughout the discourse to accept Plato’s grammatical analysis of all sentences into noun and verb, which indeed applies to the proposition as a sentence but does not give its particular elements. The first part of the work confines itself strictly to noun and verb, or the form of proposition called secundi adjacentis. Afterwards (chap. 10) proceeding to the opposition of propositions, he adds the form called tertii adjacentis, in a passage which is the first appearance, or rather adumbration, of the verb of being as a copula. In the form secundi adjacentis we only get oppositions, such as the following:—
| man is—man is not not-man is—not-man is not |
In the form tertii adjacentis the oppositions, becoming more complex, are doubled, as follows:—
| man is just—man is not just man is non-just—man is not non-just not-man is just—not-man is not just not-man is non-just—not-man is not non-just. |
The words introducing this form (δταν δὲ τὸ ἔστι τρίτον προσκατηγορῆται, chap. 10, 19 b 19), which are the origin of the phrase tertii adjacentis, disengage the verb of being (ἔστι) partially but not entirely, because they still treat it as an extra part of the predicate, and not as a distinct copula. Nor does the work get further than the analysis of some propositions into noun and verb with “is” added to the predicated verb; an analysis, however, which was a great logical discovery and led Aristotle further to the remark that “is” does not mean “exists”; e.g. “Homer is a poet” does not mean “Homer exists” (De Int. chap. 11).
How then did Aristotle get further in the logical analysis of the proposition? Not in the De Interpretatione, but in the Prior Analytics. The first adumbration was forced upon him in the former work by his theory of opposition; the complete appearance in the latter work by his theory of syllogism. In analysing the syllogism, he first says that a premiss is an affirmative or negative sentence, and then that a term is that into which a premiss is dissolved, i.e. predicate and subject, combined or divided by being and not being (Pr. An. i. 1). Here, for the first time in logical literature, subject and predicate suddenly appear as terms, or extremes, with the verb of being (τὸ εἶναι) or not being (τὸ μὴ εἶναι) completely disengaged from both, but connecting them as a copula. Why here? Because the crossing of terms in a syllogism requires it. In the syllogism “Every man is mortal and Socrates is a man,” if in the minor premiss the copula “is” were not disengaged from the predicate “man,” there would not be one middle term “man” in the two premisses. It is not necessary in every proposition, but it is necessary in the arrangement of a syllogism, to extricate the terms of its propositions from the copula; e.g. mortal—man—Socrates.
This important difference between the De Interpretatione and the Prior Analytics can only be explained by supposing that the former is the earlier treatise. It is nearer to Plato’s analysis of the sentence, and no logician would have gone back to it, after the Prior Analytics. It is not spurious, as some have supposed, nor later than the De Anima, as Zeller thought, but Aristotle in an earlier frame of mind.
Moreover we can make a history of Aristotle’s thought and gradual composition thus:
(1) Earlier acceptance in the De Interpretatione of Plato’s grammatical analysis of the sentence into noun and verb (secundi adjacentis) but gradually disengaging the proposition, and afterwards introducing the verb of being as a third thing added (tertium adjacens) to the predicated verb, for the purpose of opposition.
(2) Later logical analysis in the Prior Analytics of the proposition as premiss into subject, predicate and copula, for the purpose of syllogism; but without insisting that the original form is illogical.