Chapter 13

THE NATURE OF CLASSIFICATION, AND THE FIVE PREDICABLES.

It is merely an accident when general names are names of classes of real objects: e.g. The unity of God, in the Christian sense, and the non-existence of the things called dragons, do not prevent those names being general names. The using a name to connote attributes, turns the things, whether real or imaginary, into a class. But, in predicating the name, we predicate only the attributes; and even when a name (as, e.g. those in Cuvier's system) is introduced as a means of grouping certain objects together, and not, as usually, as a means of predication, it still signifies nothing but the possession of certain attributes.

Classification (as resulting from the use of general language) is the subject of the Aristotelians' Five Predicables, viz. _Genus_, _Species_, _Differentia_, _Proprium_, _Accidens_. These are a division of general names, not based on a distinction in their meaning, i.e. in the attributes connoted, but on a distinction in the class denoted. They express, not the meaning of the predicate itself, but its relation (a varying one) to the subject. Commonly, the names of any two classes (or, popularly, the classes themselves), one of which includes all the other and more, are called respectively _genus_ and _species_. But the Aristotelians, i.e. the schoolmen, meant by _differences in kind_ (_genere_ or _specie_) something which was in its nature (and not merely with reference to the connotation of the name) distinct from _differences_ in the _accidents_. Now, it is the fact that, though a fresh class may be founded on the smallest distinction in attributes, yet that some classes have, to separate them from other classes, no common attributes except those connoted by the name, while others have innumerable common qualities (from which we have to select a few samples for connotation) not referrible to a common source. The ends of language and of classification would be subverted if the latter (not if the former) sorts of _difference_ were disregarded. Now, it was these only that the Aristotelians called _kinds_ (_genera_ or _species_), holding _differences_ made up of _certain_ and _definite_ properties to be _differences_ in the _accidents_ of things. In conformity with this distinction--and it is a true one--any class, e.g. negro as opposed to white man, may, according as physiology shall show the _differences_ to be infinite or finite, be discovered to be a distinct _kind_ or _species_ (though not according to the naturalist's construction of _species_, as including all descended from the same stock), or merely a subdivision of the _kind_ or _species_, Man. Among _kinds_, a _genus_ is a class divisible into other _kinds_, though it may be itself a species in reference to higher _genera_; that which is not so divisible, is an individual's _proximate kind_ or _infima species_ (_species prædicabilis_ and also _subjicibilis_), whose common properties must include all the common properties of every other real _kind_ to which the individual can be referred.

The Aristotelians said that the _differentia_ must be of the _essence_ of the subject. They vaguely understood, indeed, by the _essence_ of a thing, that which makes it the _kind_ of thing that it is. But, as a _kind_ is such from innumerable qualities not flowing from a common source, logicians selected the qualities which make the thing be what it is called, and termed these the essence, not merely of the _species_, but, in the case of the _infima species_, of the individual also. Hence, the distinction between the predicables, Differentia, Proprium, and Accidens, is founded, not on the nature of things, but on the connotation of names. The _specific difference_ is that which must be added to the connotation of the _genus_ to complete the connotation of the _species_. A _species_ may have various _differences_, according to the principle of the particular classification. A _kind_, and not merely a class, may be founded on any one of these, if there be a host of properties behind, of which this one is the index, and not the source. Sometimes a name has a technical as well as an ordinary connotation (e.g. the name Man, in the Linnæan system, connotes a certain number of incisor and canine teeth, instead of its usual connotation of rationality and a certain general form); and then the word is in fact ambiguous, i.e. two names. _Genus_ and _Differentia_ are said to be of the essence; that is, the properties signified by them are connoted by the name denoting the _species_. But both _proprium_ and _accidens_ are said to be predicated of the species _accidentally_. A proprium of the species, however, is predicated of the species necessarily being an attribute, not indeed connoted by the name, but following from an attribute connoted by it. It follows, either by way of demonstration as a conclusion from premisses, or by way of causation as effect from cause; but, in either case, _necessarily_. Inseparable accidents, on the other hand, are attributes universal, so far as we know, to the species (e.g. blackness to crows), but not _necessary_; i.e. neither involved in the meaning of the name of the species, nor following from attributes which are. Separable accidents do not belong to all, or if to all, not at all times (e.g. the fact of being born, to man), and sometimes are not constant even in the same individual (e.g. to be hot or cold).