I heard that many disciplines whose names end with the letter “s” as if they were plural actually came from Greek/Latin words of plural form (e.g. mathematics from mathematika). It seems like, however, some disciplines like astronomy or philosophy also had Greek origins and their original words also ended with the letter “a”.
I have no knowledge of Greek or Latin; does it just mean that those Greek words are not plural? Or are there some reasons why such disciplines get to obtain English names with no “s” at the end?