I have sometimes heard it said that the shita of the Rambam is that it's asur to be stupid and/or believe stupid or foolish things. Clearly the Rambam's general approach puts a high value on rationalism and logic, but does he actually write anywhere that believing something foolish is prohibited? If so, I'm curious to know the source and what exactly he says. As a bonus question: if that really is his opinion, how has it been applied in practice by later poskim?
Maimonides hated all superstition with a passion, but made allowances to set people's minds at ease. He wrote:
A person bitten by a scorpion or serpent may whisper a charm over the wound even on the Sabbath, in order to settle his mind and to strengthen his heart. The thing is of no avail whatsoever, but, since he is in danger, he is permitted to do it, so he won't feel troubled. Those who whisper upon a wound a charm, consisting of verses from the Torah, or who read such verses over a child to save it from fear, or who place beside an infant a Torah scroll or tefillin to make him sleep, are not only guilty of superstition, but are amongst those who deny the Torah. They treat the words of the Torah as mere bodily medicine, whereas they are spiritual medicine. [Rambam, On Idolatry, 2:11-12]
I think the first half of the Moreh's third section makes a stronger statement:
According to the Rambam, all of halakhah is for the following three purposes:
1- To train someone away from foolishness and idolatry (including getting away from being enslaved to emotion and desire, which motivates our fooling ourselves with justifications), 2- to teach the Truth, and 3- to maintain an orderly society to give a person the time and mental clarity to accomplish the first two.
My overall impression of that section of the book is buttressed by this quote from III ch. 31 (tr. Friedlander, because it's in the Public Domain and well, good enough):
But the truth is undoubtedly as we have said, that every one of the six hundred and thirteen precepts serves to inculcate some truth, to remove some erroneous opinion, to establish proper relations in society, to diminish evil, to train in good manners or to warn against bad habits. All this depends on three things: opinions, morals, and social conduct. We do not count words, because precepts, whether positive or negative, if they relate to speech, belong to those precepts which regulate our social conduct, or to those which spread truth, or to those which teach morals. Thus these three principles suffice for assigning a reason for every one of the Divine commandments.
So he doesn't just say that is is assur (prohibited) to believe in foolish things, but that issurim and chiyuvin (prohibitions and obligations) can all be explained as ways to get one away from foolishness and to Truth.
(Although we will see later,in the last part of the last chapter, that the need to embrace the Truth is to act as Hashem appears to us as acting out of an awareness that one is following His Ways.)