What is the permanent element in thought?—It can easily be found in language—its external manifestation. Logic tells us that the expression of thought involves always a subject and predicate. Think what you please, say what you please, and your thought or assertion consists of a subject and predicate—positive or negative—joined by the copula, _is_. “Man lives” is equivalent to “man is living.” “Man” and “living” are joined by the word “is.” If we abstract all content from thought, and take its pure form in order to see the permanent, we shall have “is” the copula,—or putting a letter for subject and attribute, we shall have “_a_ is _a_,” (or “_a_ is _b_,”) for the universal form of thought. The mental act is expressed by “is.” In this empty “is” we have the category of pure Being, which is the “summum genus” of categories. Any predicate other than _being_ will be found to contain being _plus_ determinations, and hence can be subsumed under being. We shall get new light on this subject if we examine the ordinary doctrine of _explanation_. III. In order to explain something, we subsume it under a more general. Thus we say: “Horse is an animal;” and, “An animal is an organic being,” &c. A definition contains not only this subsumption, but also a statement of the specific difference. We define _quadruped_ by subsuming it, (“It is an animal”) and giving the specific difference (“which has four feet”). As we approach the “summum genus,” the predicates become more and more empty; “they become more _extensive_ in their application, and less _comprehensive_ in their content.” Thus they approach pure simplicity, which is attained in the “summum genus.” This pure simple, which is the limit of subsumption and abstraction, is pure Being—Being devoid of all determinateness. When we have arrived at Being, subsuming becomes simple identifying—Being is Being, or _a_ is _a_—and this is precisely the same activity that we found self-consciousness to consist of in our first analysis, (I.) and the same activity that we found all mental acts to consist of in our second analysis, (II.). IV. Therefore, we may affirm on these grounds, that the “summum genus,” or primitive category, is the Ego itself in its simplest activity as the “is” (or pure _being_ if taken substantively). Thus it happens that when the Mind comes to cognize an object, it must first of all recognize itself in it in its simplest activity,—it must know that the object _is_. We cannot know anything else of an object without presupposing the knowledge of its _existence_.