Rambam on Demons and Segulos (updated)
The Gemara is replete with discussions about demons, in both aggadic and halachic contexts. In my monograph, Wrestling with Demons: A History of Rabbinic Attitudes to Demons, I documented how most authorities accepted the reality of demons (and for understandable reasons). Several authorities, however, rejected the existence of demons, notwithstanding the fact that Chazal believed in their existence. The most prominent of these was Rambam. While many religious authorities did not accept that Rambam denied the existence of demons, this appears to have been because they could not accept it, due to their own religious convictions - I have yet to discover a scholar who does not believe in demons, and yet thinks that Rambam did believe in them. But there were certainly plenty of authorities who recognized that Rambam denied the existence of demons - most significantly, the Vilna Gaon.
The topic of demons is of some relevance to the topic of Chazal and science. After all, here we have Rambam and other Rishonim denying the existence of creatures whose existence was attested to by Chazal. For most of us, however, when considering Rambam's attitude to Chazal and science, it's not especially significant to look at his view on demons. The reason for this is that Rambam already wrote explicitly in the Guide that "You must, however, not expect that everything our Sages say respecting astronomical matters should agree with observation, for mathematics were not fully developed in those days; and their statements were not based on the authority of the Prophets, but on the knowledge which they either themselves possessed or derived from contemporary men of science." There is every reason to presume that if this is what Rambam felt about astronomy - an area of science with great relevance to Torah and halachah - all the more so would this be true of other areas of science.
On the other hand, for Rabbi Moshe Meiselman, Rambam's position on demons would be extremely relevant. This is because Rabbi Meiselman denies that the quotation above from the Guide demonstrates that Rambam believed Chazal to be limited to the science of their era. He insists - extremely unreasonably, as I have demonstrated previously - that Rambam was only saying that Chazal erred in a few limited cases of astronomy. With everything else that Chazal said, Rabbi Meiselman claims, Rambam believes that Chazal were correct.
Furthermore, the existence of demons, as the Vilna Gaon makes clear, is inherently related to other supernatural phenomena such as segulos. Rabbi Meiselman devotes an entire chapter to the topic of Rambam and segulos, which he states to be of relevance to the topic of Torah and science, since it demonstrates Rambam's views regarding non-scientific phenomena. Rabbi Meiselman claims that Rambam did indeed believe in the efficacy of segulos - and he further argues that this is the "mainstream view." However, while Rashba - who was passionately committed to the existence of non-scientific phenomena - did indeed ascribe this view to Rambam, numerous others understood that Rambam denied the efficacy of segulos.
In a recent post at Torah Musings, Rabbi Gil Student states that although Rabbi Meiselman claims that Rashba's interpretation of Rambam “was adopted by many other authorities,” Rabbi Meiselman does not name any, and Rabbi Student does not know of any. Rabbi Student further points out that despite Rabbi Meiselman's claim that the view of Radvaz, that Rambam denied the efficacy of segulos, was “not adopted by any other major interpreter of the Rambam," it was actually echoed by no less than the Chida and the Vilna Gaon, amongst others.
I do not know if Rabbi Meiselman considers himself wiser than the Vilna Gaon and all the other authorities who stated that Rambam denied the existence of demons and segulos. However, even if he considers them to be wrong, he should still acknowledge the existence of their views.
The reason why he doesn't acknowledge their views is obvious. It's because they fatally flaw his entire book, which is dedicated to claiming that no mainstream figure ever held that Chazal could be wrong in their claims about the world.
UPDATE - In the comments section, it was pointed out to me that the Vilna Gaon accuses Rambam of falsely reinterpreting the Gemara, not of disputing it. Thus, the Vilna Gaon's statement would not be relevant to claims about Rishonim stating that Chazal were wrong.
Did Rambam believe himself to be disputing Chazal, or did he convince himself that Chazal also did not believe in demons? I'm still looking into it. On the one hand, it's impossible to imagine how one could convince oneself that Chazal did not believe in demons. On the other hand, Rambam certainly convinced himself of very strange things regarding the Neviim.
If we consider the situation with astrology, Rambam claims that only a few sages believed in it (whether he was being diplomatic, or actually genuinely thought so, is an interesting question that is difficult to answer). However, even if only a few sages mistakenly believed in astrology or demons, that is still fatal to Rabbi Meiselman's claims.