Summary: As a follow-up to Rav Yonasan Eibeshitz's rocket ship, or rather space elevator, and Rashi's rocket ship, I present the Rambam's iron airship. In condemning the over-use of imagination, the Rambam accidentally demonstrates its power.
Post: Continuing from the summary, in the Rambam's Shmonah Perakim (page 3 in this PDF), the Rambam discusses the imaginative faculties, and warns against over-reliance upon them.
"The element of imagination: this power [relates to the faculty of memory and makes it possible to] recall the impression of various incidents after they are no longer perceptible by the senses. [This faculty can also] combine and separate different [recollections]. Thus, this power has the potential to develop [a notion] that was never thought of previously and that cannot be understood, based on one’s previous perception. For example, one might imagine an iron ship flying through the air, or a man with his head in the heavens and his feet on the earth, beast with one thousand eyes and the like. Many impossible things of this nature can be conceived by the power of imagination. In this matter, the sect of medabrim erred grossly and established a false foundation with regard to the distinctions concerning what must exist, what may exist and what cannot exist. For they thought - or imagined - that anything that could be imagined [by a person] could actually exist. They did not realize that this power has the potential of making a combination of various matters that cannot possibly exist, as explained above."
The Rambam appears to be correct in terms of this limitation. That is, it certainly seems correct that man is capable of conceiving the impossible, and that man imagining something is no proof that it either may or must exist. Yet, it seems that his examples were poorly picked, because he imagined things that we now know are indeed possible. An iron ship flying through the air is simple an airplane. And a beast with 1000 eyes calls to mind the compound eye of most insects.