We have said that the first notion of the intellect is being, that this notion corresponds to the most basic and most universal determination of things. We have further made plain that being comprises two aspects complementary of each other, essence and existence; whence its definition as "the something which is." But though all this is true, it does not yet touch the real problem concerning the notion of being. A universal notion, being yet differs from every other universal notion; it has characteristics all its own and cannot be dealt with in the common manner of universals. In addition, it labors under an inner tension or opposition arising from its very structure; which again sets it apart from every other notion. Hence the problem, for the metaphysician, concerning the structure of the concept 10 of being. We must take stock of this problem and then see how St. Thomas and his tradition resolve it.
Recall, as a beginning, the universality of the concept of being, the most comprehensive notion that can be conceived. Everything in reality, whether actual or possible, falls under being, hence under the concept of being. How can the totality of things so diverse be united under one concept which will include them all, in their diversities as well as in their identities?
A comparison from logic will show what is at stake. In logic, a genus is reducible to a species, and vice versa. Assume, as illustration, that animals can be classified under two principal species, vertebrate and invertebrate. All animals, then, belong to the one genus animal and are divided into two species by their respective difference, vertebrate and invertebrate. Thus, in the language of logic, a genus is reduced or contracted to its various species by the addition of various specific differences. What makes such a procedure possible and permissible is that the differences concerned are not actually contained in the genus. Animal as such (i.e. abstracted from this or that kind) is neither vertebrate nor invertebrate. The genus, which unites all animals under one concept, is thus the principle of unity or identity; and the specific difference, which separates them into classes, the principle of diversification. But the point is that diversification of the genus is introduced from the outside, as it were; genus in short does not contain, actually, the specific differences.
Suppose we now try this maneuver with the notion of being, and see what happens. Being is multitudinous; wherever we turn we are surrounded by things, and things are beings. Yet one concept embraces them all, hence the concept has a certain unity; its meaning, in other words, must somehow apply to all the things it covers, else it would not be, as in fact it is, predicable of all. "When, for example, I say "this table is," "this color is," etc. I mean that the attribute or circumstance denoted by is occurs, proportionately, in the table, in the color, and indeed in everything which is. However, I am also cognizant that the table has not the same mode of existence as the color (the color is in the table, the table is not in the color). This diversity inherent to the notion of being is further accentuated when we predicate being of transcendental (in general, immaterial) realities, and never more so than when it is predicated of God. God, too, is; but is his being commensurate with lesser realities? And if not, what is it that will differentiate him from other beings, and these again from each other? Will it be something that is not being? Impossible; for what is not being is nothing, and nothing differentiates nothing. Plainly, the differences of being must also be some kind of being. But then, how can they still be differences?
Whatever the solution, one thing is even now clear: the concept of being cannot be diversified 11 in the manner of a genus, since there are no real differences that are not themselves being. The question thus boils down to creating distinction within a concept without resorting beyond the concept; or, to importing differences which are somehow there already. This, precisely, is the problem concerning the structure of the concept of being, a problem that can lead astray. For too much stress on the unity of being jeopardizes the diversity of it and of its concept; and overemphasis of diversity endangers the unity of it and of its concept. Ultimately, the first path leads to monism, a philosophical desert such as detained the Eleatics of old; and monism is only the foreside of pantheism. The second travels to the other extreme, so much plurification (differentiation) that being is robbed of community, failing which there fails the material of any organic thought. How, then, escape this dilemma, the Sylla and Charybdis of the metaphysical Odyssey? We escape it through the doctrine of analogy.