The most capital distinction, however, which is to be found among the Categories is that of Essence or Substance from all the rest. This is sometimes announced as having a standing _per se_; as not only logically distinguishable, but really separable from the other nine, if we preserve the Aristotelian list of ten,[106] or from the other three, if we prefer the reduced list of four. But such real separation cannot be maintained. The _Prima Essentia_ (we are told) is indispensable as a Subject, but cannot appear as Predicate; while all the rest can and do so appear. Now we see that this definition is founded upon the function enacted by each of them in predication, and therefore presupposes the fact of predication, which is in itself a Relation. The Category of Relation is thus implied, in declaring what the First Essence is, together with some _predicabilia_ as correlates, though it is not yet specified what the _predicabilia_ are. But besides this, the distinction drawn by Aristotle, between First and Second Essence or Substance, abolishes the marked line of separation between Substance and Quality, making the former shade down into the latter. The distinction recognizes a more or less in Substance, which graduation Aristotle expressly points out, stating that the Species is _more_ Substance or Essence, and that Genus _less_ so. We see thus that he did not conceive Substance (apart from attributes) according to the modern view, as that which exists _without_ the mind (excluding _within_ the mind or _relation_ to the mind); for in that there can be no graduation. That which is without the mind, must also be within; and that which is within must also be without; the subject and the object correlating. This implication of within and without understood, there is then room for graduation, according as the one or the other aspect may be more or less prominent. Aristotle, in point of fact, confines himself to the mental or logical work of predication, to the conditions thereof, and to the component terms whereby the mind accomplishes that act. When he speaks of the First Essence or Substance, without the Second, all that he can say about it positively is to call it _Unum numero_ and indivisible:[107] even thus, he is compelled to introduce unity, measure, and number, all of which belong to the two Categories of Quantity and Relation; and yet still the First Essence or Substance remains indeterminate. We only begin to determine it when we call it by the name of the Second Substance or Essence; which name connotes certain attributes, the attributes thus connoted being of the essence of the Species; that is, unless they be present, no individual would be considered as belonging to the Species, or would be called by the specific name.[108] When we thus, however, introduce attributes, we find ourselves not merely in the Category of _Substantia_ (_Secunda_), but also in that of _Qualitas_. The boundary between