Of substances constituted by nature some are ungenerated, imperishable, and eternal, while others are subject to generation and decay. The former are excellent and divine, but less accessible to knowledge. The evidence that might throw light on them, and on the problems which we long to solve respecting the, is furnished but scantily by sensation; whereas respecting perishable plants and animals we have abundant information, living as we do in their midst, and ample data may be collected concerning all their various kinds, if only we are willing to take sufficient pains. Both departments, however, have their special charm. The scanty conceptions to which we can attain of celestial things give us, from their excellence, more pleasure than all our knowledge of the world in which we live; just as a half glimpse of persons that we love is more delightful than an accurate view of other things, whatever their number and dimensions. On the other hand, in certitude and completeness our knowledge of terrestrial things has the advantage. Moreover, their greater nearness and affinity to us balances somewhat the loftier interest of the heavenly things that are the objects of the higher philosophy.
Aristotle, Parts of Animals, Book 1, Part 5 (644b21-645a4).
Saint Thomas Aquinas, following Aristotle, distinguishes three types of speculative knowledge therefore: physica, mathematica and metaphysica, depending on how much the objects of each of these sciences is dependent on matter and change.
Cf. Saint Thomas Aquinas, Commentary on the De Trinitate of Boethius, q. 5, art. 1, resp.