Thus Plato taught, and Aristotle cordially agreed with him. See the seventh book Of the Republic, and the Posterior Analytics. Plato, Aristotle and the schoolmen based their notions of science upon the exact sciences of arithmetic, geometry, and formal logic, these being the first sciences developed. With us, the name of science has been well-nigh monopolised by the study of physical nature. Physical objects certainly belong to the class of things contingent: they are, but might not be. This is true: but the physicist does not consider his science perfect till he has attained to the knowledge of the laws of physical necessity which govern the operations of those contingent things. Observation and experiment are preliminary steps to science. And physical necessities belong to the region of the eternal. A substance, such as chlorine, must act in this or that way under those conditions, if ever at any time it is to be at all. This is an eternal truth. This is exactly St Thomas's teaching, when he says: "The understanding attains to science of perishable things, only in so far as they are imperishable, -- that is to say, in so far as they become to the mind universals." Cf. I, Chap. LXVII, with notes.

Of God and His Creatures: 2.55