Guess I can appreciate why they thought this way
There's an early observation, popularized by Plato, that we perceive all sorts of weird things that don't actually exist in reality. Like, we've never seen a straight line (which is infinitely thin & not jagged), not to mention the number 7. Such ideal objects are only mental & don't exist in reality
So it's like there's a world of ideal forms, that we can only observe mentally. Nowadays, a philosopher like Chomsky
says it comes from our genetic endowment; back then, people like Plato didn't know about genetics
In Descartes' time, they had engineers, like watchmakers. Nowadays with computing, we imagine the universe as a computer simulation. Well back then, they imagined a clockwork universe. Modelled by the "mechanical philosophy", where things affected each other by bumping into each other. (This was before Newton's "spooky occult" action-at-a-distance forces like gravity.) Well, the mind didn't work that way. So he said the mind was maybe made of some crazy thing other than matter. ("Mind-body dualism", the ghost in the machine...)
Well, Newton eventually
exorcized the machine & kept the ghost...
btw, dunno about anyone else, but I think Descartes' translations from
earlymoderntexts.com totally rocks. Contains many of his letters, it's like reading his email inbox
One problem was having to deal with the Church, which was censorious at the time; he assumed his letters were probably read. I remember Chomsky mentioned some writings of Descartes were propaganda to please the Church; can't remember which. Maybe the Meditations, at least some of them?
Maybe his more serious work was "Principles of Philosophy". But again, don't recall