17. The Predicables. -- Human thought is abstractive and unitive. It represents the reality of nature by means of an assemblage of abstract notes susceptible of being universalized. How do these notes (predicates) contribute to the formation of a complete intelligible object (subject)? What relation exists between the subject and the predicate? In other words, by what right is the latter "predicated" of the former?

There are various predicables or modes of predicability:

(1) Necessary, because essential predicables. -- Certain characteristics constitute the essence of the thing, which makes the thing what it is (quod quid est, to ti ên einai), and without which it could not exist or be conceived: such are animality and reason in man.

(2) Necessary, though non-essential predicables. -- Other attributes do not constitute the substance, but necessarily result from it. In an invariable manner they interpret -- develop -- the constitutive perfection of the subject: these are called its properties (proprium, idion.).

(3) Contingent or accidental predicables. -- Others, again, have a contingent connection with the essence: these are called contingent accidents (contingit ut sint, sumbebêkos), or, more briefly, accidents.^{1}

Essential predicables are subdivided: The object of the intelligence is not the individual essence, but the specific essence represented by different abstract and universal concepts. The term species (eidos) designates the sum of the abstract and universal notes which constitutes an essence as the human mind knows it.^{2}

Certain of these constitutive notes of a species are at the same time applicable to other species; these are called generic, they constitute the kind, or genus some are proper to it and differentiate it from other species of the same genus, and these form the specific difference (diaphora).

Hence three distinct essential predicables; the species and its two parts, the genus and the specific difference.

Add to these three predicables property and accident, and we have altogether five predicables, or catagoremes.

The properties (idion) are the determinations which, without being of the essence of the thing, necessarily follow from the essence and, consequently, cannot be separated from it.

A note is said to be proper to a given species when it belongs exclusively to that species, universally to all individuals of that species, and constantly to each one of them. "Proprium dicitur quod convenit soli alicui speciei, omni et semper."

Thus the radical aptitude for learning letters is proper to man; incorruptibility is proper to immaterial substances; limitation is proper to creatures. In this, the only rigorous acceptation, property, has the same extension as essence.

When a characteristic does not combine the three conditions here stated, it is no longer, rigorously speaking, a property; it is no longer co-extensive with essence.

Nevertheless, though in a lesser sense, it justifies the appellation when it presents one or two of the three distinctive notes of property: A characteristic which belongs exclusively to a specific type, even though it do not belong either universally or constantly to the representatives of the species, is, in this sense, a property: thus, it is proper to man to be a physician, to be a geometrician.

Similarly, a characteristic found in all the individuals of a species, and always, but not belonging to them exclusively, may be called a property: in this sense, says Porphyry, it is proper to man to be a two-legged animal.

Such, too, is a characteristic which is common to all the representatives of the species and to them only, but temporarily: Thus, according to Porphyry, it would be proper to man -- to every man and to men only -- to grow grey in old age.

The common accidental quality, accident as opposed to idion, accidens commune as opposed to proprium), may be defined in a negative way: the quality which is not a property in the strict sense of the word. In a positive way Porphyry defines it: An accident is a quality to the presence or absence of which the essence of the subject is indifferent (Accidens est quod adest et abest praeter subjecti corruptionem).

The common accidental quality, Porphyry adds, is sometimes constant, sometimes belongs to the subject only intermittently. We may say of the animal that it sleeps; black plumage may be attributed to the crow constantly.

From this we see that care must be taken not to confound the quality, even the constant quality, with the property.

The mere observation of facts does not suffice to effect the discernment of a property. This discernment, we shall see later, forms the object of scientific induction and calls for the employment of experimental methods.

{1} Evidently, we must take care not to confound the (ontological) accident which is contradistinguished from the substance -- whether it have contingent or necessary attachments to the substance -- with the (logical) accident which is immediately contradistinguished from the essence on the one hand, and, on the other hand, from the accidents called properties. What is predicated as accidental of one subject may be predicated as essential of another.

{2} Species in the logical acceptation here defined must not be confounded with the same term in the sense attached to it by naturalists -- that of a collection of individuals capable of indefinite reproduction among themselves.