Was Rabbeinu Avraham a Solipsist?
Continuing [1A] with the Discourse's analysis of R. Ammi, Rabbi Meiselman spots another contradiction. The Rambam, in his Letter on Astrology, lists three sources of knowledge: (1) sense data, (2) reason and (3) knowledge "that a man receives from the prophets or from the righteous."  But, according to Rabbi Meiselman, the Discourse gives no validity to authority as a source of knowledge, hence a contradiction with the Rambam's third source of knowledge. 
Similarly, in confirming Chazal's health recommendations, he says that they are "essential to health as has been validated by investigation and the medical practice of the physician". Here, Rabbeinu Avraham does attribute Chazal as the source of knowledge, as well as the confirmation by contemporary medicine. Moreover, Rabbeinu Avraham nowhere tells the reader to embark on his own investigation, but to rely on what he himself writes in the Discourse. 
In claiming a contradiction, Rabbi Meiselman most likely refers R. Ammi's hypothetical rejection of Yehoshua. By Rabbi Meiselman's argument, R. Ammi could not reject the authority of Yehoshua over R. Ammi's logical arguments, while simultaneously respecting Yehoshua as a source of knowledge. However, we have already shown that the Chasam Sofer explicitly agrees with the Discourse's understanding of the Talmud.  Yet, the Chasam Sofer certainly accepted knowledge from the righteous or a prophet as a source of knowledge.
Rabbi Meiselman may also refer to the following statement earlier in the Discourse (translation mine):
You should know that anyone who desires to support a well-known opinion  and to give respect to its author and accept his opinion without any investigation into the matter as to its veracity, that this is an improper methodology and prohibited...However, this statement does not claim that knowledge cannot be derived from a knowledgeable authority. The statement merely emphasizes that one should not blindly accept such statements purely based on who said them "without any investigation" (for example by consulting with other authorities). Hence, when the Jewish Sages claim that the Sun goes behind the sky-shell at night, we do not accept this view simply because of their greatness. Likewise, in modern times, we don't accept geocentrism which was the near-universal opinion of both the ancient and medieval greats, both Jewish and Gentile. Instead, we understand that the science of astronomy has made great advances since then and that these statements need to be re-examined.
Thus, Rabbi Meiselman sets forth a false dichotomy: either an authority is considered reliable in all that he says on a subject or no knowledge can be derived from him. He makes a similar statement in TCS (pg. 225) in an attempt to prove that the Chazal's medical knowledge was received knowledge (ellipses mine):
Chazal, however, were not physicians...Why then should they be considered a "source" at all in this area [medicine]? Why should they not be considered amateurs quoting hearsay? Evidently ... Chazal's advice was of a permanent nature because it was not based on experimentation but upon a Torah source. [7a]To bring this misconception this into sharper relief, consider the field of ethnomedicine, the study of traditional medicine as practiced by various cultures. One important aspect of ethnomedicine is the discovery of new drugs based on an understanding of traditional remedies. (For example, see this list of drugs derived from ethnomedicine). In this case, traditional remedies are a important and irreplaceable source of knowledge, but such knowledge is not accepted blindly. Rather it forms the starting point for research and verification . We can simultaneously place a high value on a given source of knowledge yet retain a skeptical approach.
Perhaps Rabbi Meiselman's argument is a bit different: Rabbeinu Avraham favors reason over authority, while the Rambam lists sense data, reason and authority as three sources of knowledge with no particular ranking. Does the Rambam favor reason over authority as Rabbeinu Avraham implies? The answer is clearly "yes".
To begin with, one can simply ask: if the only three sources of knowledge are sense data, reason and knowledge received from an authoritative figure, whence does the authoritative figure derive his knowledge? In the Rambam's hierarchy of knowledge, the third source, authority, is derived from the other two (sense data and reason). While the Rambam implies this hierarchy, do we find that he makes this explicit? The answer again is "yes".
In his Treatise on Logic, the Rambam gives a slightly different categorization of the sources of knowledge:
- Sense data (e.g. this object is black)
- First ideas (e.g. if a=b and b=c, then a=c)
- Conventions (e.g. it is important to thank someone that helps you)
- Traditions (e.g. knowledge received from trusted person or group)
In contrast, knowledge derived from conventions or traditions are not universally recognized and result in a knowledge with a lower level of certainty. Knowledge derived from traditions form the art or rhetoric and may involve weaker forms of reasoning such as reasoning by analogy, which is disallowed in the art of demonstration. 
Thus, the Rambam does put reason above authority in the hierarchy of knowledge sources. But we can ask: how does this apply practically to statements of Chazal? Are their scientific statements to be treated in the same way, as described by the Discourse?
The Rambam answers this quite clearly in his Letter on Astrology. While most of the letter is devoted to rejecting the general accepted belief in astrology, at the end of the letter he addresses another difficulty: if the Talmud accepts astrology, how can we reject it? He offers a number of possible grounds for rejecting such Talmudic statements, including the possibility that the Talmudic statement is simply in error. He concludes with the following aphorism (discussed earlier), which asserts the priority of reason is such circumstances: "A man should never cast his reason behind him, for the eyes are set in front, not in back". 
We see once again that Discourse aligns quite well with the Rambam. In the next post, we'll address some other discrepancies claimed by Rabbi Meiselman between the Discourse and the Rambam.
Comments are both welcome and encouraged. I'll make every effort to address any questions or arguments posted in the comments.
[1A] After a short interlude far away from lions.
 It is unclear why Rabbi Meiselman needs to quote the Rambam here. No human being can navigate the world without accepting, at least provisionally, knowledge obtained from others. Any time that you consult, say, a doctor, an architect or or a plumber, you are relying on the fact that they have knowledge that you don't have through your own perception or your own reason. Only a solipsist can claim to rely entirely on knowledge directly acquired through their own perception or reason.
In fact, the Rambam is not making any kind of novel or innovative statement here. He is simply reminding the reader that any belief should have some source, either primary or secondary; the mere fact that some claim (in the case, the efficacy of astrology) is written in a book or is believed by the masses is not valid grounds for acceptance. Thus he writes
The great sickness and the "grievous evil" (Eccles. 5:12, 15) consist in this: that all the things that man finds written in books, he presumes to think of as true—and all the more so if the books are old. And since many individuals have busied themselves with those books and have engaged in discussions concerning them, the rash fellow's mind at once leaps to the conclusion that these are words of wisdom...Were it true that the Discourse completely rejects expert the expertise of others as a source of knowledge, there would be no need to find a contradictory statement from the Rambam.
 We are not in duty bound to defend the opinions of the sages of the Talmud, concerning medicine, physics and astrology, as right in every respect simply because we know the sages to be great men with a full knowledge of all things regarding the Torah, in its various details. Although it is true that in so far as knowledge of our Torah is concerned, we must believe the sages arrived at the highest stage of knowledge, as it is said (Deu. 17, 11.) "In accordance with the instructions which they may instruct thee, etc.," still it is not necessarily so concerning any other branch of knowledge.
 "Since we find that the sages themselves had said, concerning nledical knowledge that the opinion of such and such a Rabbi did not prove to be true, as for instance, The eagle-stone (Sabbath fol. 66b), or other things mentioned."
 These two paragraphs should go without saying, but are included in the interest of thoroughness. We can dismiss any author by interpreting what he says to be nonsensical. And an author whow never relies on outside knowledge is being nonsensical.
 We showed that this was the view of the Rambam as well, but our reasoning doesn't depend on that argument.
 Perhaps instead "the opinion of a well known person".
[7a] Obviously, Rabbeinu Avraham's explanation is much more straightfoward: some of Chazal's medical knowledge was well-found and some not.
 For example, see J Ethnobiol Ethnomed. 2007; 3: 25. "The present zootherapeutic study describes the traditional knowledge related to the use of different animals and animal-derived products as medicines by the Saharia tribe reside in the Shahabad and Kishanganj Panchayat Samiti's of Baran district of Rajasthan, India. ... Further studies are required for experimental validation to confirm the presence of bioactive compounds in these traditional remedies and also to emphasize more sustainable use of these resources." This is just a single sample found randomly using a google search.
 The same applies to ideas, first and second; by second ideas I mean such as geometric theorems and astronomic calculations, which are all true, because they may all be demonstrated by premises, most of which come close to the first ideas. In like manner, all the results of experience, e. g., that scammony is a cathartic and gall-nut causes constipation, are also true. Whatever becomes known through one of these three truthful channels the logicians call apodictic. After these preliminaries, you must know that every syllogism both of whose premises are apodictic, we call a demonstrative syllogism; and the making of these syllogisms and a knowledge of their conditions constitute what we call the art of demonstration.
 When, however, one or both premises of the syllogism belong to conventions, we call it a dialectic syllogism; and the making of these syllogisms and a knowledge of their conditions constitute the art of dialectics. When one or both premises of the syllogism belong to traditions, we call it a rhetorical syllogism; and the making of these syllogisms and a knowledge of their conditions constitute the art of rhetoric. [...] In general, however, the demonstrative syllogisms do not use analogy under any circumstances, nor do they use induction except under certain conditions; but the art of dialectics does use general induction; and the art of rhetoric uses the analogical syllogism.
 The summary of the matter is that our mind cannot grasp how the decrees of the Holy One, blessed be He, work upon human beings in this world and in the world to come. What we have said about this from the beginning is that the entire position of the star gazers is regarded as a falsehood by all men of science. I know that you may search and find sayings of some individual sages in the Talmud and Midrashim whose words appear to maintain that at the moment of a man's birth, the stars will cause such and such to happen to him. Do not regard this as a difficulty, for it is not fitting for a man to abandon the prevailing law and raise once again the counterarguments and replies (that preceded its enactment). Similarly it is not proper to abandon matters of reason that have already been verified by proofs, shake loose of them, and depend on the words of a single one of the sages from whom possibly the matter was hidden. Or there may be an allusion in those words; or they may have been said with a view to the times and the business before him. (You surely know how many of the verses of the holy Law are not to be taken literally. Since it is known through proofs of reason that it is impossible for the thing to be literally so, the translator [of the Aramaic Targum] rendered it in a form that reason will abide. ) A man should never cast his reason behind him, for the eyes are set in front, not in back... (Letter on Astrology)