Rambam has always been known as a revolutionary, controversial figure - and with good reason. I am always amused at the reaction of certain non-Charedim when, after they express derision at Charedi intolerance of Rambam-type views, I inform them of Rambam's views on topics such as providence, the World-to-Come, and reward and punishment. Suddenly they're not so tolerant themselves!
Traditionally, the traditionalist response to Rambam's revolutionary ideas has been to write them off as influenced by Greek philosophy and therefore not kosher, to explain that he only taught these ideas for outreach and did not subscribe to them, or to claim that his writings must actually be interpreted as referring to mysterious mystical concepts (always a fail-safe last resort!).
But recently I have noticed a new approach by anti-rationalists. It is to claim that, properly understood, Rambam never actually said anything controversial! Furthermore, the claim is that Rambam never accepted anything from Aristotle against the mesorah! I kid you not.
The fact is that not only were Rambam's philosophical views heavily influenced by Greco-Muslim philosophy, but, as Dr. Marc Shapiro has demonstrated at length in Studies in Maimonides and his Interpreters, so was the Mishneh Torah. And it's not only the view of modern academic scholars that Rambam's worldview was, to a large extent, the product of Greco-Muslim philosophy rather than a tradition from his rebbe'im. The Vilna Gaon famously commented that Rambam was “led astray by the accursed philosophy” to deny the existence of demons and other such phenomena. Rav Hirsch observed that Rambam's "trend of thought was Arab-Greek... Approaching Judaism from without, he brought to it views that he had gained elsewhere, and these he reconciled with Judaism." (The Vilna Gaon and Rav Hirsch were also influenced by non-traditional sources, but not quite as blatantly as Rambam.)
All this is fairly obvious to any honest and serious student of Rambam and history. But I just noticed that Rambam himself explicitly notes that his approach to Maaseh Merkavah (Ezekiel's chariot) - one of the major topics in the Guide for the Perplexed - was not based on any received tradition, but rather he developed it himself, based on his philosophical studies:
No divine revelation came to me to teach me that this was the intent of the matter, nor have I received my belief in this respect from any teacher; rather, I have been informed by what I learned from Scripture and the utterances of our Sages, together with the philosophical principles which I have adopted, that the matter is as such, without doubt. But it is possible that the matter is otherwise, and the meaning is different. (Introduction to Part III of the Guide, translation based on Schwartz edition)
As Chaim Kreisel writes in an article that can be freely downloaded, Rambam's subsequent discussion "does not leave the slightest doubt that the Chariot of Ezekiel is a parable for the structure of the heavens as depicted by scientists and philosophers" - including the Ptolemaic spheres and the four elements.
Now, Rambam himself truly believed that this is what Yechezkel was talking about. He may have also believed that Chazal knew this; I'm not sure. (In some cases, such as his denial of demons and astrology, he knew full well that he was going against Chazal, but here it's not so clear.) But he was certainly aware that he was reconstructing the original intent based on what he had absorbed from Greek philosophy - not transmitting a mesorah from his rebbe'im. And does anyone today believe that this is what Maaseh Merkavah was really about - the Ptolemiac spheres and the four elements?
This is not to denigrate the Rambam, G-d forbid. He was trying to understand things as best as possible, just as we all do. But there is a valuable lesson that he taught in general and specifically with this topic: that one should accept the truth from wherever it comes - even from outside our community.