The operation of the human intellect is twofold, however; first, simple perception, 'simple apprehension,' the 'simple gaze of indivisibles' and second, composition and division or judgment. In considering the principles of human knowledge it is therefore necessary to distinguish simple principles from complex principles or axioms. It is evident, however, that being is absolutely first of all complex as well as incomplex principles. "That which first falls under apprehension is being, the understanding of which is included in all things whatsoever (...) man apprehends. Therefore, the first indemonstrable principle is the same cannot be affirmed and denied at the same time, which is based on the constitutive intelligibility of being and non-being: and on this principle all others are based.". (shrink)

According to St. Thomas Aquinas, "that which is said to exist through any nature is called a suppositum or subject of that nature. For example, that which has the nature of horse is said to be a subject or suppositum of equine nature." Subjects or supposita, moreover, occupy all the room there is in the Thomistic universe, since existence belongs properly only to individual subjects. These may be simple, as in the case of separate intelligences or composite as in the (...) case of inanimate and animate substances: "existence belongs properly to subsisting things, whether they be simple, as in the case of separate substances, or composite, as in the case of material substances. For the act of existing belongs properly to that which has existence: that is, to that which subsists in its own existence.". (shrink)