For each Heaven illuminates visible things; and thus each Science illuminates the things intelligible.

And the third similitude is the inducing of perfection in the things so inclined.

Of which induction, as to the first perfection, that is, of the substantial generation, all the philosophers agree that the Heavens are the cause, although they attribute this in different ways: some from the Movers, as Plato, Avicenna, and Algazel; some from the stars themselves, especially the human souls, as Socrates, and also Plato and Dionysius the Academician; and some from celestial virtue which is in the natural heat of the seed, as Aristotle and the other Peripatetics.

Thus the Sciences are the cause in us of the induction of the second perfection; by the use of which we can speculate concerning the Truth, which is our ultimate perfection, as the Philosopher says in the sixth book of the Ethics, when he says that Truth is the good of the intellect.

Because of these and many other resemblances, it is possible to call Science, Heaven.
Now it remains to see why it is called the third Heaven.