The vast majority, therefore, of those who reject the Thomistic doctrine on individuation, support the view that what individuates any nature or substance is simply the whole reality, the total entity, of the individual. This total entity of the individual, though really identical with the specific nature, must be conceived as something positive, superadded to the latter, for it involves a something which is logically or mentally distinct from the latter. This something is what we conceive as a _differentia individua_, after the analogy of the _differentia specifica_ which contracts the concept of the genus to that of the species; and by Scotists it has been termed “_haecceitas_” or “thisness”. Without using the Scotist terminology, most of those scholastics who reject the Thomist doctrine on this point advocate the present view. The individuality or “thisness” of the individual substance is regarded as having no special principle in the individual, other than the whole substantial entity of the latter. If the nature is simple it is of itself individual; if composite, the intrinsic principles from which it results—_i.e._ matter and form essentially united—suffice to individuate it.
In this view, therefore, the material principle of any individual man, for example, is numerically and individually distinct from that of any other individual, _of itself_ and independently of its relation either to the formative principle or to quantity. The formative principle, too, is individuated _of itself_, and not by the material principle which is really distinct from it, or by its relation to this material principle. Likewise the union of both principles, which is a substantial mode of the composite substance, is individuated and rendered numerically distinct from all other unions of these two individual principles, not by either or both these, but by itself. And finally, the individual composite substance has its individuation from these two intrinsic principles thus individually united.
It may be doubted, perhaps, whether this attempt at explaining the real, individual “manifoldness” of what is “one” for intellect, _i.e._ the universal, throws any real light upon the problem. No doubt, every element or factor which is grasped by intellect in its analysis of reality—matter, form, substance, accident, quantity, nay, even “individuality” itself—is apprehended as abstract and universal; and if we hold the doctrine of Moderate Realism, that the intellect in apprehending the universal attains to reality, and not merely to a logical figment of its own creation, the problem of relating intelligibly the reality which is “one” for intellect with the same reality as manifestly “manifold” in its concrete realizations for sense, is a genuine philosophical problem. To say that what individuates any real essence or nature, what deprives it of the “oneness” and “universality” which it has for intellect, what makes it “this,” “that,” or “the other” incommunicable individual, must be conceived to be simply the whole essential reality of that nature itself—leaves us still in ignorance as to why such a nature, which is really “one” for intellect, can be really “manifold” in its actualizations for sense experience. The reason why the nature which is one and universal for abstract thought, and which is undoubtedly not a logical entity but a reality capable of actual existence, can be actualized as a manifold of distinct individuals, must be sought, we are inclined to think, in the relation of this nature to a material principle in alliance with quantity which is the source of all purely numerical, “space and time” distinctions.