Miscellanies 8.6.21

The differentia too is added to definitions, holding the place of a sign for the definition. At any rate, when “capable of laughter” is added to the definition of man, it makes the whole: a rational, mortal, land-dwelling, walking animal, capable of laughter. For the things added to the definition by way of differentia are signs of the things proper to it, but do not indicate the nature of the things themselves. Indeed, they say that the differentia is the rendering of a property; and insofar as what has the differentia differs from all others, a differentia that belongs to it alone and is convertible with the thing in definitions, one must necessarily take the first genus as primary and subsisting. In the longer definitions, then, the multitude of species found through the ten categories is taken; in the shortest, the leading ones among the proximate species, taken together, signify the substance and nature of the thing; and the shortest consists of three, the genus and two of the most necessary species. This is done for brevity. We say, then: “Man is the rational animal capable of laughter.” And one must also take in what belongs peculiarly to the thing defined, either its own excellence or its own work, and some other such things. Definition, then, though it explains the substance of a thing, cannot comprehend the nature of the thing exactly, but it makes the substance plain through the most proper species; and definition holds the substance, as it were, in the manner of a quality.