The perfectly intelligible sense in which Aristotle and the scholastics designate the universal a substance, the sense of moderate realism, according to which the universal constitutes, and is identical with, the essence of the individual “person” or “thing,” is entirely different from the sense in which many exponents of modern monistic idealism conceive the universal as the substance _par excellence_, the _ens realissimum_, determining, expressing, evolving itself in the individual phenomena of mind and of nature, which would be merely its manifestations.(279) (_b_) The divisions of substance into spiritual and corporeal, of the latter into inorganic and organic, of these again into vegetative and animal, and finally of animal substances into brute animals and human beings,—offer no special difficulties. All purely natural or rational knowledge of the possibility and nature of purely spiritual substances is based on the analogy of our knowledge of the human soul, which, though a spiritual substance, is not a pure spirit, but is naturally allied with matter in its mode of existence. The individual human being offers to human experience the sole example of the sufficiently mysterious conjunction and combination of matter and spirit, of the corporeal mode of being and the spiritual mode of being, to form one composite substance, partly corporeal and partly spiritual. (_c_) This in turn suggests the division of substances into _simple_ and _composite_. The latter are those which we understand to be constituted by the natural and substantial union of two really distinct but incomplete substantial principles, a formative, determining, specifying principle, and a material, determinable, indifferent principle: such are all corporeal substances whether inorganic, vegetative, sentient, or rational. The former, or simple substances, are those which we understand to be constituted by a sole and single substantial principle which determines and specifies their essence, without the conjunction of any material, determinable principle. We have no direct and immediate experience of any _complete_ created substance of this kind; but each of us has such direct experience of an _incomplete_ simple substance, _viz._ his own soul; while we can infer from our experience the _existence_ of other incomplete simple substances, _viz._ the formative principles of corporeal substances, as also the _possibility_ of such complete simple substances as pure spirits, and the actual _existence_ of the perfectly simple, uncreated substance of the Infinite Being.