Galen
Well, according to the esteemed and veritable ‘Wikipedia’ (sic):
Galen (Greek: Γαληνός, Galēnos; Latin: Claudius Galenus; AD 129–ca. 200 or 216) of Pergamum was a prominent ancient Greek physician, whose now discredited theories dominated Western medical science for over a millennium. The forename “Claudius”, absent in Greek texts, was first documented in texts from the Renaissance.
Here’re some fine examples of Galen’s famous now-discredited theories:
“The heart has two chambers with numerous connections” (Galen must’ve dissected a cadaver with a serious case of congenital septal defects, and thought that it was normal)
“The brain is a large clot of phlegm, from which the psychic ‘pneuma’ is formed by a rhythmical pump” (whatever the second part of the statement meant….. but brain as a large clot of phlegm? Oh please, the brain deserves more credit than to be equated with phlegm!)
“The small intestine is long because it saves having to eat all the time” (Nice try, Galen… nice try.)
As ludicrous as these statements may sound, it is worrying that these ideas were thought of as correct for nearly a millenium. One might be tempted to wonder how our ‘medical knowledge’ of today will be viewed by our future generations in a thousand years from now…..