_substantial_ unity in the latter as their fundamental principle, a unity resulting from the _union of incomplete substances_. This latter is undoubtedly the view of St. Thomas, of practically all the medieval scholastics, and of most scholastics in modern times. Nor do we see any sufficient reason for receding from it, or admitting the modern distinction between substance and nature. And if it be objected that the view which admits the reality of incomplete substances and substantial change is as much a preconception as what we have called the atomic view of substance, our answer is, once more, that since we have no intellectual intuition into the real constitution of the substances which constitute the universe, since we can argue to this only by observing and reasoning from their activities on the principle _Operari requitur esse_, the evidence alone must decide which view of these substances is the correct one. Does the evidence afforded us by a scientific analysis of all the functions, inorganic, vegetative, sentient and rational, of an individual man, forbid us to conclude that he is one complete substance, resulting from the union of two incomplete substantial principles, a spiritual soul and a material principle? and at the same time compel us to infer that he is one complete nature resulting from the union of a plurality of principles supposed to be complete as substances and incomplete as natures? We believe that it does not; nor can we see that any really useful purpose is served by thus setting up a real distinction between substance and nature. From the evidence to hand it is neither more nor less difficult to infer unity of substance than unity of nature in the individual. The inference in question is an inference from facts in the phenomenal order, in the domain of the senses, to what must be actually there in the noumenal order, in the domain of nature or substance, a domain which cannot be reached by the senses but only by intellect. Nor will any imagination images which picture for us the physical fusion or coalescence of material things in the domain of the senses help us in the least to conceive in any positive way the mode in which incomplete natures or substances unite to form a complete nature or substance. For these latter facts belong to the domain which the senses cannot reach at all, and which intellect can reach only inferentially and not by direct insight.