Metaphysics differentiates between abstract and concrete, as well as between universal and particular, entities. This is best seen with an example:

1. Abstract Universal: greenness.
2. Concrete Particular: a green apple.
3. Abstract Particular: individual numbers. They are neither concrete objects nor universals, being particular entities which do not themselves occur inside time or space.
4. Concrete Universal: notion crucial to Christian theology and also to G.W.F. Hegel´s school of Absolute Idealism. In short, a concrete universal is something that connects everything together, which the absolute idealists identified as the Absolute (an all-inclusive mental entity). Christian theology, in contrast with the idealists, emphasized its Tri-Une God (and its incarnation in Jesus Christ) as the true concrete universal that solved the problem of unity (universality) and multiplicity (concreteness).

Regarding the Absolute, Christian theologians such as Cornelius Van Til argued that it presents problems that from a non-Christian starting point of view (axioms and presuppositions) are unsolvable.

For example, the Absolute, when viewed as a unifying element, appears to swallow its source of plurality, our world ruled by chance. In addition, the existence of the latter seems to undermine the “absoluteness” of the Absolute (being its necessary counterpart which actualizes the former´s potentiality).

In Christianity, Van Til emphasized, both unity and plurality are seen as equally fundamental ^B. For the Christian God, the One and the Many are equally ultimate, God Himself (the Trinity) being the underlying harmony between them. With creation having been  modelled after its Creator, this is believed to be reflected in our world, too.

The very possibility of us having knowledge, in fact, depends on the co-primacy of unity and multiplicity.

This is so because, if only the plurality of the particular has primacy, we end up with unrelated entities of which nothing can be known in principle, as they become abstract particulars with nothing in common, a world unto themselves.

Furthermore, if absolute primacy is given instead to the unity of the universal, we have the same problem. Abstracting things into universal categories leads to them losing part of what makes them unique. If we aim for absolute abstract unity, we will obtain complete homogeneity instead. The loss of the particular means the loss of everything that makes each thing (or being, e.g., a person) what (or who) it is.

This also makes knowledge impossible as knowledge implies the existence of differentiated entities, contrary to absolute sameness (e.g., it is not possible to think about “personhood” without thinking about a specific person).

Either way we end up being unable to make any distinction between particulars, so again, nothing can be known in principle. What appeared to be two opposing positions turned out to be two sides of the same coin.

Therefore, even in our world, in order to be able to know anything, our reality must be such that its unity and plurality are related yet there is difference.¹Dialectics is the contrary belief that states that we should decide between one horn or the other of any specific dilemma. Its underlying presupposition is that difference means the same as contradiction or opposition, with one option always being better than the rest. It excludes the possibility of the existence of many equally good options. If something is good, everything else is bad by virtue of being different.

If this is true for our plane of existence, it is to be expected that it is also true for the higher reality that gave birth to it, as a superior reality cannot have additional limitations compared to a lower one.

Christian apologists defend that for the Tri-Une God this is accomplished at all levels: in our reality (as we have mentioned through the example of knowledge), in the Godhead (the Trinity) and in Jesus Christ, the Incarnation of the Logos and recapitulation of all that exists, who Himself is seen as the greatest possible symbol (icon) of the unity of the universal and the particular.

In the next section we will expand on these matters and delve into the historical answers provided to this tension between unity and multiplicity, the universal and the particular, in what became known as the problem of the One and the Many.

A. “The ‘Self‘ is the transcendent and permanent principle of which the manifested being, the human being, for example, is only a transient and contingent modification, a modification which, moreover, can in no way affect the principle […]

The ‘Self’, as such, is never individualized and cannot become so, for since it must always be considered under the aspect of the eternity and immutability which are the necessary attributes of pure Being, it is obviously not susceptible of any particularization, which would cause it to be ‘other that itself’.

Immutable in its own nature, it develops the indefinite possibilities which it contains within itself, by a relative passing from potency to act through an indefinite series of degrees.

Its essential permanence is not thereby affected, precisely because this process is only relative, and because this development is, strictly speaking, not a development at all, except from the point of view of manifestation, outside of which there is no question of succession, but only of perfect simultaneity, so that even what is virtual under one aspect, is found nevertheless to be realized in the ‘eternal present’.”

― René Guénon (1925). Man and His Becoming According to the Vedanta. Ch.2

B. “In seeking for an answer to the One-and-Many question, philosophers have admittedly experienced great difficulty. The many must be brought into contact with one another. But how do we know that they can be brought into contact with one another? How do we know that the many do not simply exist as unrelated particulars? The answer given is that in such a case we should know nothing of them; they would be abstracted from the body of knowledge that we have; they would be abstract particulars.

On the other hand, how is it possible that we should obtain a unity that does not destroy the particulars? We seem to get our unity by generalizing, by abstracting from the particulars in order to include them into larger unities. If we keep up this process of generalization till we exclude all particulars, granted they can all be excluded, have we then not stripped these particulars of their particularity? Have we then obtained anything but an abstract universal?”

― Cornelius Van Til (1955). The Defense of the Faith. P&R Publishing, p. 25-26