St. Thomas has an extremely broad account of “natural science”- all objects of sensation, or which are verified to exist by appeal to sensory evidence are in natural science. What else is there? How can an object even be present to us that is not present under these conditions? Leaving aside mathematical objects, metaphysical ones are reached by noticing a certain contrariety in the sensible things themselves. We observe, for example, that all the goods we know are limited. But the intelligible character of good is contrary to limitation, so far as we cannot love finite goods precisely so far as they are limited. Limitation marks the point where a certain good ceases and is cut off from other goods, and we cannot love anything because it is cut off from loveable things. Unlimited goodness thus becomes simply a clarification of what we have always meant by good: it was not the first thing called good, but it is most of all what the term means. Once such a principle gets admitted, it lets a good deal of other things in as well: truth, for example, is the good of the intellect, and so unlimited truth is simply a clarification of what truth is; and similar considerations apply to perfection, beauty, unity, nobility, causality, existence, etc, and then pour out to include the powers reality that correspond to.
Metaphysics is therefore a certain clarification of what we have known about a certain class of objects we have spoken of our whole lives; it is a distillation of things that have always been known, a distillation that requires that we have not encountered them in their pure form from the beginning. Christ’s correction of the rich young man “why do you call me good? No one is good but God alone” is not simply hyperbole; it is more like a grammatical correction from someone who knows what a human being means by the term “good”.
The object of metaphysics is thus simultaneously unthinkable and yet what was known all along, just as a good is never present to our thought without limitation (and certainly was not from the beginning), though we have always excluded this limitation from what we meant by good. The first “never”, however, is a temporary condition- one that applies only so long as our intellect is pressed into the service of keeping a corruptible body alive.
It is this sort of articulation of metaphysics that throws light on the notion that life is absurd if not for God; the sense is that if we clarify that good must be unlimited, then logically- though this may not be possible existentially- we are left only with the options that either an absolute unlimited good exists (and that we can somehow attain him) or all of our desires are ultimately absurd (or that God exists and we cannot attain to him in any way). St. Thomas gives a proof that we should conclude to one side of this contradiction, but other proofs have been given that seek to persuade to the other side.