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). CHAPTER VIII. DEFINITION.