Aristotle's On Interpretation Ch. 10. segment 19b19: To convey a thing about the subject of an assertion that we express as a noun we predicate “is” or “is not” of the subject in addition to that noun
(19b19) of Ch. 10: To convey a thing about the subject of an assertion that we express as a noun we predicate “is” or “is not” of the subject in addition to that noun
An assertion is a composite speech instance. To make an assertion we combine simple speech instances such as the noun and the verb. For example, the assertion “a man runs” combines the noun “man” with the verb “runs”. This acknowledged, an assertion always carries either a verb, an indefinite verb or one of their inflections. It never comprises only nouns and indefinite nouns. To illustrate, “a man not-fast” is no assertion until we add e.g. “is” or “is not” or “was” to it. One of the reasons for this is that verb-type utterances function to situate the meaning of their assertion in time. This Aristotle establishes in On Int. Ch. 5 17a10 - 17a15.
Now, when we formulate an assertion we signify one thing about one thing. The thing we signify we call a predicate. The thing of which we signify a predicate we call a subject. For example, in the assertion “a man runs”, the verb “runs” is the predicate. This we signify of the noun “man” which is, in this case, the subject. That being said, not everything we may signify of a subject is a verb. In fact, as the Categories taught us, many of the things we signify of a subject we express as nouns. Here, we count secondary substances such as “animal”, qualities such as the colour “white” and relatives such as “fast” among others.
In On Int. Ch. 10 19b19 Aristotle explains to us how to formulate assertions in which the one thing we signify about the subject is a noun. Namely, he instructs us to predicate either the verb “is”, its indefinite counterpart “is not”, or an inflection such as “was” as a third thing, in addition to the noun we want to signify of the subject. When we add “is” we formulate an affirmation, i.e we assign the meaning of the predicate noun to the subject. With “is not” we formulate a negation. That is, we exclude the meaning of the predicate noun from the subject. When we formulate our assertion with an inflection such as “was” or “will be” we situate its meaning in the past or the future. By way of illustration, to affirm “animal” of “man” we say “man is an animal”, to deny “fast” of “a turtle” we say “a turtle is not fast”, to affirm “white” of “this wall” in the past we say “this wall was white”.
We thereby observe that while some assertions combine only two things, i.e. they carry only two constitutive elements (e.g. “a horse runs” combines “runs” with “a horse), others combine three things (e.g. “a horse is fast” combines “fast”, “is” and “a horse”). In the latter case, two of the constitutive elements are nouns or indefinite nouns. The third is either the verb “is”, its indefinite form “is not” or one of its inflections such as “was” or “will not be”.
Key points: (i) When we want to say something of a subject which we typically express with a noun or indefinite noun, we form our assertion by predicating either the verb “is”, its indefinite form “is not”, or one of their inflections as an additional third element in our assertion. (ii) With “is” we affirm the meaning of the noun-part of the predicate of our subject. With “is not” we deny it. With an inflection we situate it outside the present time, i.e. either in the past or future.
