The Baraisa Kinyan Torah (AKA Avos ch. 6), par. 5 lists 48 characteristics needed to fully acquire Torah. One of the traits in this list is emunas hakhamim (faith in the sages). What exactly is this extent of this faith? What topics does it include?
The Midrash Shmuel writes:
That one believes that all that the sages say is as if it was given to Moses at Sinai. On this it is written, "You shall not turn aside to the right or to the left from the sentence which they pronounce upon you" (Deut. 17:11). If you do not believe in even one thing, the secrets of Torah will not be revealed to you. In the end, you will become a Saducee because one sin leads to another.The clear implication is that the sages retain authority on all matters. One must believe everything they say on any topic. I hesitate to call it a concept of complete infallibility, and prefer to refer to it as universal authority. Any idea the sages pronounce is to be accepted as authoritative, regardless of the topic.
The Mahazor Vitry has a much more limited explanation of emunas hakhamim. It records simply and briefly: "That one believes their words, unlike Saducees and Boethusians." The Saducees and Boethusians disputed the sages on matters of oral law. The implication from the Mahazor Vitry is that emunas hakhamim requires us to have faith in the chain of the tradition of the oral law. Pronouncements from sages about the oral Torah are considered authoritative and binding. However, there is no indication that emunas hakhamim goes beyond the chain of tradition of the oral law.
R. Avraham Farissol (cited in the Me'orei Or commentary in the Masekhes Avos Im Peirush Ha-Gra Ha-Mevu'ar) explains similarly:
That they believe in the explanations of the sages who explain the laws, and they believe in their decree for the needs of the time and place. Even though one can question slightly the matter or the decree, it is proper to believe in it.He also understands emunas hakhamim to be referring to the area of halakhah. There is no indication that it would include philosophy, theology or science.
More recently, the Tiferes Yisrael writes of emunas hakhamim:
That one does not believe everything that one hears, for this is the trait of the fool who believes everything. Rather, one believes the words of the sages about the wisdom of the Torah, even if one's intellect does not comprehend them.According to these last three commentators, emunas hakhamim is limited to accepting the authority of the sages regarding the transmission and integrity of the Torah. Rather than giving the sages universal authority on all matters, it confers on them legal authority, meaning authority on matters regarding the law.
It seems to me that this second approach is connected to that of those authorities (Ramban, R. Avraham ben Ha-Rambam, etc.) who sanction occasionally rejecting an aggadic text but would never allow such leeway in regard to an halakhic passage. This is not too say, though, that great scholars may be summarily rejected. A wise man does not lose his wisdom when the topic of conversation shifts. Rather, his level of expertise on a particular subject might be less than complete, perhaps even without his knowledge, and this should be taken into account. Recent rabbinic pronouncements about science come to mind. On this subject, it is worth reviewing R. Aharon Lichtenstein's words (here).