It may be profitable at this juncture to introduce the notion of a concrete universal. 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?
As Christians we hold that there is no answer to these problems from a non-Christian point of view. We shall argue this point later; for the nonce we introduce this matter in order to set forth the meaning of the notion of the concrete universal. The notion of the concrete universal has been offered by idealist philosophy in order to escape the reductio ad absurdum of the abstract particular and the abstract universal. It is only in the Christian doctrine of the triune God, as we are bound to believe, that we really have a concrete universal. In God’s being there are no particulars not related to the universal and there is nothing universal that is not fully expressed in the particulars.
– Cornelius Van Til, The Defense of the Faith p.48-49