Now that we know how humans fit into the broad spectrum of existing things in this universe; and now that we have classified humankind in relation to the other things; we can move on to getting knowledge about human nature itself. This is what we will call the philosophy, or science, of human nature.
For Aquinas, a science was defined as “knowledge of things through their causes…It reaches its ideal, not simply when it records observable connections in nature and calculates them in mathematical terms, but rather when it accounts for observable phenomena and the properties of things by bringing into light their intelligible relations to their causes.”
Furthermore, as Aristotle says, “every demonstrative science has three elements: (1) that which it posits, the subject genus whose essential attributes it examines; (2) the so called axioms, which are primary premisses of its demonstration; (3) the attributes, the meaning of which it assumes.”
Science is, therefore, also concerned with examining what its object is. Most sciences do not go so deep as to talk about being as being, however, they do talk about qualified being. Metaphysics, is the science which studies being as being.
Therefore, philosophy, for Aquinas, was just as much a science as the natural sciences, and more certain about its findings. Each science has its own proper object, as we mentioned above, and has its own particular way of coming to knowledge, both about the object, and about the causes of that object.
In order to complete our discussion of the nature of human-beings in more detail we will look at the metaphysical constitution of all existing things, and of human-beings in particular.
Thomas Aquinas, The Division and Method of the Sciences, 4th ed., trans. Armand Maurer (Toronto: PIMS, 1986), Introduction, ix.