St. Thomas distinguishes two different ways of developing an idea: we either develop the idea on its own level of universality, or we develop some of the various less universal ideas that fall under it. Taken in the first way, we would develop the idea of animal by considering animals as animals (When do we have an animal as opposed to something else? What are the main sorts of animals? At what point did animals emerge in history? ) taken in the second way, we’d develop the ideas that we have of elephants or apes or mosquitoes or men, etc. Again, these are two ways of developing one idea, and so there is going to be an obvious dialectic between the two approaches. Nevertheless, the first approach is decidedly “horizontal”, that is, it stays on a certain level of universality and always stays on it, while the second approach develops an idea on a completely different plane of universality. In visual terms, the first develops an idea horizontally at one level of universality, the second develops the idea vertically by filling out lower levels of universality. Nevertheless, on this second approach, we still develop these lower ideas themselves “horizontally”, and so the two levels of analysis never intersect.
And so the development of any idea can be compared to drawing one-dimensional lines across a two-dimensional plane or possible analyses, that is, any line of inquiry has an infinite penumbra of forgotten reality, or indefinite analyses that are omitted as not only unconsidered but unconsiderable. Any line of analysis is only a line drawn across a plane of being – and this is true even of the line of analysis called “metaphysics”. It is certainly true of any line of analysis called “science”. Since “being” is the whole field of intelligible reality, then being is hidden to an indefinite degree in any line of inquiry, no matter how developed – even if it is infinitely developed by an ideal mind – so far as that mind is knowing by concepts more or less universal.
(N.B. What is crucial to the image is the division between the two lines of inquiry, so far as a higher one never meets up with or leads to the conclusions of the lower one. The “space” we visualize between the two lines of inquiry is of no value to the example, since degrees of universality are not infinite.)
We might even push the infinity of this penumbra further by noting that even our consideration here is limited to modes of universality. If we considered the methods of approach, we add a third dimension to things: for we not only can consider them by methods of analysis but also by artistic methods: poems, novels, myths; by demonstration or dialectics; in their historical or philosophical character, etc.. These are different sorts of universals manifesting the idea in a different mode, and so the inquiry into being is now comparable to a three-dimensional solid that we still traverse with one-dimensional lines of inquiry.
(N.B. One counter-point to all this would be St. Thomas’s idea that human knowledge can be filled out and perfect, and that it in fact reached such a state in Christ. A central point of this claim is that there is a single term of inquiry: the species specialissima of things. A full account or critique of this idea of the hiddenness of things would have to take these into account.)