Voltaire (1694-1778) wrote a satirical account of a medieval university's oral examination on medicine: the examiner asks why morphine puts people to sleep, and the student confidently replies that morphine has a "dormative essence". This is a bit like saying that things with an essence of gravity fall towards the earth, whereas things with an essence of levity float towards the sky. The examiner proudly accepts that answer and bestows upon the student the title of doctor.
This was very funny to anyone educated in the time Voltaire was writing, since they would have known that morphine puts people to sleep because... it has round molecules with no sharp edges...
Edit: correction below, thank you thaumasiotes.
It was Molière (<1622 - 1673), who wrote a play that featured an apothecary (explaining the functioning of opium to laymen), but no examinations.
https://web.archive.org/web/20101218031844/http://www.medsch...
People will probably find it offensive but it does explain a lot about why the stuff being taught is almost always not questioned.
This post in particular:
https://web.archive.org/web/20101211180021/http://www.medsch...