Commentary on Peter Lombard’s Sentences (1252-1256), Book 2, Distinction 12, Question 1, Article 2 (trans. J. F. McCarthy):
For certain things are per se the substance of the Faith, as that God is three and one, and other things of this kind, in which no one is authorized to think otherwise. Thus the Apostle says in Galatians 1 that if an angel of God preached diversely from what he had taught, let him be anathema. But certain things (pertain to the faith) only incidentally (per accidens), inasmuch, that is, as they are handed down in Scripture, which faith supposes to have been promulgated under the dictation of the Holy Spirit. And these things can without danger remain unknown by those who are not held to be knowledgeable about the Scriptures, for example, many items of history. In these things even the Fathers have thought differently and have explained the Scriptures in different ways. So, therefore, with regard to the beginning of the world, there is something which pertains to the substance of the Faith, namely, that the world was created to begin with. And this all the Fathers agree in saying. But how and in what order it was made does not pertain to the Faith except per accidens, inasmuch as it is presented in Scripture, the truth of which the Fathers retained in their varying explanations as they arrived at different conclusions.
Commentary on Aristotle’s De Caelo (On the Heavens) (1272-1273), Book 2, Lecture 12, no. 451 (trans. F. R. Larcher and P. H. Conway):
For the earliest astronomers took Saturn as the outermost planet and after it Jupiter; Mars they put third, the Sun fourth, Venus fifth, Mercury sixth, and the Moon seventh. But the astronomers of Plato's and Aristotle's time changed this order as to the Sun, by putting it immediately above the Moon and under Venus and Mercury, which positing Aristotle follows here. Later on, Ptolemy corrected this order of the planets by showing that what the earlier astronomers said was truer, and this is the opinion held by current astronomers.
Secondly, we must keep in mind that certain "anomalies," i.e., irregularities, appear with respect to the motions of the planets. For the planets seem to be now swifter, now slower, now stationary, now retrogressing. Now this does not seem to be appropriate to heavenly motions, as is evident from what has been said above. Therefore, Plato first proposed this problem to an astronomer of his time, named Eudoxus, who tried to reduce these irregularities to a right order by assigning diverse motions to the planets; a project also undertaken by later astronomers in various ways. Yet it is not necessary that the various suppositions which they hit upon be true — for although these suppositions save the appearances, we are nevertheless not obliged to say that these suppositions are true, because perhaps there is some other way men have not yet grasped by which the things which appear as to the stars are saved. Aristotle nevertheless uses suppositions of this kind, in what regards the quality of the motions, as true.
The third thing that must be considered is that not as many kinds of irregularities appear with respect to the sun and moon as with the other planets: for the sun and moon never appear to be stationary or to undergo retrograde movement as do the other planets, but present only swiftness and slowness.
Summa Theologiae (1265-1273), Prima Pars, Question 32, Article 1, Reply to Objection 2 (trans. English Dominican Fathers):
Reason may be employed in two ways to establish a point: firstly, for the purpose of furnishing sufficient proof of some principle, as in natural science, where sufficient proof can be brought to show that the movement of the heavens is always of uniform velocity. Reason is employed in another way, not as furnishing a sufficient proof of a principle, but as confirming an already established principle, by showing the congruity of its results, as in astrology the theory of eccentrics and epicycles [i.e. the theory of geocentrism] is considered as established, because thereby the sensible appearances of the heavenly movements can be explained; not, however, as if this proof were sufficient, forasmuch as some other theory might explain them.
