I am studying the $S$ duality between 11D supergravity and type IIA superstring theory from Adel Bilal’s paper. The author says:
Of course, eleven dimensional supergravity is not expected to yield a consistent quantum theory. It should only be the low-energy limit of some consistent theory, baptised M-theory.
I don’t understand what this really means. What is a “consistent quantum theory”? And how do we know that 11 dimensional supergravity is the low energy limit of M theory?