Of this mixed character, partly logical, partly ontological, is the first distinction set forth in the Categoriæ--the distinction between matters _predicated of_ a Subject, and matters which are _in_ a Subject--the Subject itself being assumed as the _fundamentum_ correlative to both of them. The definition given of that which is _in_ a Subject is ontological: viz., "_In_ a Subject, I call that which is in anything, not as a part, yet so that it cannot exist separately from that in which it is."[18] By these two negative characteristics, without any mark positive, does Aristotle define what is meant by being _in_ a Subject. Modern logicians, and Hobbes among them, can find no better definition for an Accident; though Hobbes remarks truly, that Accident cannot be properly defined, but must be elucidated by examples.[19] [Footnote 18: Aristot. Categ. p. 1, a. 24.] [Footnote 19: Hobbes, Computation or Logic, part i. 3, 3, i. 6, 2, ii. 8, 2-3.] The distinction here drawn by Aristotle between being _predicated of_ a Subject, and being in a Subject, coincides with that between essential and non-essential predication: all the predicates (including the _differentia_) which belong to the essence, fall under the first division;[20] all those which do not belong to the essence, under the latter. The Subjects--what Aristotle calls the First Essences or Substances, those which are essences or substances in the fullest and strictest meaning of the word--are concrete individual things or persons; such as Sokrates, this man, that horse or tree. These are never employed as predicates at all (except by a distorted and unnatural structure of the proposition, which Aristotle indicates as possible, but declines to take into account); they are always Subjects of different predicates, and are, in the last analysis, the Subjects of all predicates. But besides these First Essences, there are also Second Essences--Species and Genus, which stand to the first Essence in the relation of predicates to a Subject, and to the other Categories in the relation of Subjects to predicates.[21] These Second Essences are less of Essences than the First, which alone is an Essence in the fullest and most appropriate sense. Among the Second Essences, Species is more of an Essence than Genus, because it belongs more closely and specially to the First Essence; while Genus is farther removed from it. Aristotle thus recognizes a graduation of _more or less_ in Essence; the individual is more Essence, or more complete as an Essence, than the Species, the Species more than the Genus. As he recognizes a First Essence, _i.e._ an individual object (such as Sokrates, this horse, &c.), so he also recognizes an individual accident (this particular white colour, that particular grammatical knowledge) which is _in_ a Subject, but is not _predicated of_ a Subject; this particular white colour exists _in_ some given body, but is not _predicable of_ any body.[22]