...we are not obliged, on account of the great superiority of the sages of the Talmud, and their expertise in their explanations of the Torah and its details, and the truth of their sayings in the explanation of its general principles and details, to defend them and uphold their views in all of their sayings in medicine, in science and in astronomy, or to believe them [in those matters] as we believe them regarding the explanation of the Torah, which they had completely mastered and which it was their role to teach, as it says, "According to the Torah that they teach you" (Deuteronomy 17:11).
You can see the full section in Hebrew and English at this link. Actually, it's a pity that the idea that Chazal were not infallible in science has come to be known as "Rabbeinu Avraham's view," since, as shown in my monograph "The Sun's Path at Night," this was a normative view amongst the Rishonim. Labeling it as "Rabbeinu Avraham's view," as Rav Aharon Feldman does, downgrades it from a normative view amongst Rishonim and Acharonim to the minority view of an obscure figure. The probably cause of this attribution is that Rabbeinu Avraham's discussion of this idea is so explicit and well-known.
Another assault upon the treatise of Rabbeinu Avraham has been to claim that it is a forgery(!). This claim was made by Rav Moshe Shapiro and will soon be published by Rabbi Moshe Meiselman in his book "The Torah of Science" (along with some spectacular revisionism of the Rav). Again, it is strange that anti-rationalists take this strategy, since even getting Rabbeinu Avraham out of the way does not help with the fact that the fallibility of Chazal in scientific matters is a perfectly normative view held by dozens and dozens of Rishonim and Acharonim. But, since these rabbonim are apparently unaware of this fact, they see it as valuable to dismiss Rabbeinu Avraham's treatise as a forgery.
Needless to say, there are absolutely no serious grounds for considering it to be a forgery. In the past, I have been in touch with Professor Paul B. Fenton, who is probably the world's greatest expert on Rabbeinu Avraham, about this. Today, I noticed that he recently published a seminal article entitled "Maimonides—Father and Son: Continuity and Change" in Traditions of Maimonideanism (Ed. Carlos Fraenkel), where a footnote reveals that additional genizah fragments of the Kifâyat al-‘abidîn (Rabbeinu Avraham's original work from which the treatise on Aggadah was extracted) are continually coming to light:
One particular chapter of the Kifâya, that dealing with the interpretation of the Midrash, came to be considered as a separate composition and was thrice translated into Hebrew in the Middle Ages and in the sixteenth century, with the title Ma'amar al ha-aggadôt.(39) In recent times fragments from the original Arabic have been discovered in the genizah.(40) Let us not forget that Maimonides also intended to compose a special work on the interpretation of the midrash from a philosophical standpoint, but this desire too remained unfulfilled. Perhaps this chapter by Abraham is to be also perceived as the son’s realization of his father’s wish, although, as we shall see presently, in a specific paragraph of this section, Abraham inveighs against philosophy.
39 An ancient anonymous translation is to be found in ms. Neubauer 1649, copied in Poland in 1465. It was published and printed several times from this manuscript, for example in Kerem Hemed 2 (1836), pp. 7–61; Maimonides, Qôbez II, pp. 40–43, and recently in R. Abraham Maimonides, Milhamôt ha-shem, pp. 81–98. A second translation was made in the East in the sixteenth century by Abraham Ibn Migash (See A. Harkavy, Hadashim gam yeshanim 10 (1896), p. 87) and a third, in the same century, in the Maghreb by Vidal Sarfati of Fez in the introduction to his commentary on the Midrash rabba, Imrey yôsher, Warsaw, 1874.
40 See E. Hurwitz, Ma'amar al ’ôdôt derashôt Hazal, Joshua Finkel Memorial Volume, New York, 1974, pp. 139–168. To the fragments discovered by the latter scholar, can be added the following two Genizah fragments we have identified: Westminster College, Arabica II.39 and AIU, Paris IIA 1, which originally belonged to the same manuscript.
If you haven't yet studied Rabbeinu Avraham's treatise, the 18th of Kislev would be a great day to do so!
(Thanks for Menachem Butler for inspiring this post and showing me various source material.)