“This portion of the theory of general language is the subject of what is termed the doctrine of the Predicables; a set of distinctions handed down from Aristotle, and his follower Porphyry, many of which have taken a firm root in scientific, and some of them even in popular, phraseology.  The predicables are a five-fold division of General Names, not grounded as usual on a difference in their meaning, that is, in the attribute which they connote, but on a difference in the kind of class which they denote.  We may predicate of a thing five different varieties of class-name: A genus of the thing (γένος). A species (εἶδος). A differentia (διαφορα). A proprium (ἰδιόν). An accidens (συμβεβηκός). It is to be remarked of these distinctions, that they express, not what the predicate is in its own meaning, but what relation it bears to the subject of which it happens on the particular occasion to be predicated.”  J. S. Mill, System of Logic, i. 133.