The Rambam, practically alone among the major commentators, has the almost unique view that sheidim do not exist, and the entire idea was a simple superstition, played upon by the Sages for use parables and other concepts. Consequently, he interpreted all the Gemaros and Midrashim that mention sheidim as allegories or the like. Though some of his interpretations can be found in Moreh Nevuchim, we unfortunately do not have his explanations to all the Aggados in Shas, though he did at one time plan on compiling such a book (see his introduction to the Moreh). His son Rav Avraham seems to hold his father's views on the matter. See, for example, his introduction to Aggados, printed in most editions of Ein Yaakov. The Meiri, in his running commentary of the Gemara, makes a point of trying to explain how the Rambam may have learned these Gemaras. The vast majority of Rishonim and Acharonim, however, chief among them the Ramban, are of the opinion that sheidim do in fact exist. This is primarily based on the hundreds of references in Chazal to sheidim, which certainly seem to exist. The Ramchal in Derech Hashem explains that sheidim are somewhat physical and somewhat spiritual beings. The Tannaim and Amoraim seemed to frequently meet and even interact with sheidim. The question remains: Why do we not see or detect sheidim nowadays? The simple answer is that we are not on the spiritual level to notice such beings. My Grandfather shlita told me that Rabbi Ruderman zt"l, late Rosh Yeshivah of Ner Yisroal, suggested a different reason. The Gemara in Pesachim 112b relates that certain great Sages were on the spiritual stature to decree, as it were, that the sheidim not reveal themselves on certain days of the week. Rav Ruderman suggested, perhaps somewhat in jest, that the very fact that the Rambam thought shaidim do not exist caused, as it were, their disappearance.