We have already made it clear to you that in Aristotle’s time the number of the spheres had not been accurately established and that those who in our time count nine spheres, only count as one a globe that includes several spheres, as is clear to whoever has studied the science of astronomy. For this reason you also should not regard as blameworthy this dictum of some1 of the Sages, may their memory be blessed:2 There are two firmaments; for it is said: Behold, unto the Lord thy God belongeth the heaven, and the heaven of heavens.3 For he who says this counts the whole globe of the stars — I mean the spheres in which there are stars — as one globe, and again counts the globe of the all-encompassing sphere in which there is no star as the second globe. Consequently, he says: There are two firmaments.
Now I shall first set forth for your benefit a preface needed for the purpose that I have in view in this chapter. This preface is as follows. Know that regarding the spheres of Venus and Mercury there exists a difference of opinion among the early mathematicians about whether they are above the sun or below the sun.4 For there is no demonstration proving to us what the position of these two spheres5 is. The doctrine of all the ancients was that the spheres of Venus and Mercury are above the sun. Know this and keep it entirely present in your mind. Then Ptolemy came and decided in favor of the opinion that they were both below the sun, saying that the greatest likeness to a natural order would be manifested in the sun’s being in the middle with three planets6 above and three below. Then came latter-day groups of people in Andalusia7 who became very proficient in mathematics and explained, conforming to Ptolemy’s premises, that Venus and Mercury were above the sun. In fact, [20a] Ibn Aflaḥ of Sevilla,8 whose son I have met, has written a celebrated book about this. Thereupon the excellent philosopher Abū Bakr Ibn al-Ṣaʾigh,9 under the guidance of one of whose pupils I have read texts, reflected on this notion and showed various ways of argumentation — transcribed by us from him — by means of which the opinion that Venus and Mercury are above the sun may be shown to be improbable. However, the argument set forth by Abū Bakr is one purporting to show that this opinion is improbable, not one purporting to disprove in entirely. Whether this matter be so or not, all the early mathematicians put Venus and Mercury above the sun. For this reason they counted five spheres:10 namely, the sphere of the moon, which undoubtedly is contiguous with us; that of the sun, which is necessarily above it; that of the five planets; that of the fixed stars; and the all-encompassing sphere11 in which there are no stars. Accordingly, the number of informed spheres,12 I mean to say the spheres with forms, in which there are stars — for as is generally known from their books, the ancients called the stars forms — is four; namely, the sphere of the fixed stars, that of the five planets, that of the sun, and that of the moon; while above all of them there is one empty sphere13 in which there is no star.
Now this number is for me a very important basis for a notion that has occurred to me and that I have not seen explicitly stated by any philosopher. I found, however, in the dicta of the philosophers and the discourse of the Sages indications that drew my attention to it. I shall accordingly mention them and explain the notion in the following chapter. [20b]