Now it remains to see why it is called the third Heaven. Here it is requisite to reflect somewhat with regard to a comparison which exists between the order of the Heavens and that of the Sciences Wherefore, as has been previously described, the Seven Heavens next to us are those of the Planets; then there are two Heavens above these, the Mobile, and one above all, Quiet. To the Seven first correspond the Seven Sciences of the Trivium and of the Quadrivium, namely, Grammar, Logic, Rhetoric, Arithmetic, Music, Geometry, and Astrology. To the eighth Sphere, i.e., to the starry, correspond Natural Science, which is termed Physics, and the first Science, which is termed Metaphysics. To the ninth Sphere corresponds Moral Science; and to the Quiet Heaven corresponds Divine Science, which is designated Theology.
And the reason why this is, remains briefly to be seen. I say that the Heaven of the Moon is likened unto Grammar because it is possible to find a comparison to it. For if you look at the Moon well, two things are seen to be proper to it which are not seen in the other stars: the one is the shadow which is in it, which is no other than the rarity of its body, in which the rays of the Sun can find no end wherefrom to strike back again as in the other parts; the other is the variation of its brightness, which now shines on one side, and now on the other, according as the Sun sees it. And these two properties Grammar has: for, because of its infinity, the rays of reason can find no end in it in parts, especially of the words; and it shines now on this side, now on that, inasmuch as certain words, certain declensions, certain constructions, are in use which were not formerly, and many formerly were which again will be; as Horace says in the beginning of his book on the art of Poetry, when he says: “Many words will spring up again which have now fallen out of use.”
And the Heaven of Mercury may be compared to Logic because of two properties: that Mercury is the smallest star in Heaven, that the amount of its diameter is no more than two hundred and thirty-two miles, according as Alfergano puts it, who says that it is one twenty-eighth part of the diameter of the Earth, which is six thousand five hundred miles; the other property is, that it is more concealed by the rays of the Sun than any other star. And these two properties are in Logic: for Logic is less in substance than any other Science, for it is perfectly compiled and terminated in so much text as is found in the old Art and the new; and it is more concealed than any other Science, inasmuch as it proceeds with more sophistical and probable arguments than any other.
And the Heaven of Venus may be compared to Rhetoric because of two properties: the one is the brightness of its aspect, which is most sweet to behold, far more than any other star; the other is its appearance, now in the morning, now in the evening. And these two properties are in Rhetoric: for Rhetoric is the sweetest of all Sciences, since it principally aims at sweetness. It appears in the morning, when the Rhetorician speaks before the face of the hearer; it appears in the evening, that is, afterwards, when it speaks by Letters in distant parts.