Having imagination doesn't matter if you don't understand what geniuses came before you and what they interpreted and misinterpreted about the world. His theory upended Newtonian physics on some level, which is the practical basis for Calculus in the first place. You have to understand those concepts on a deep and abstract level to be able to challenge a dominant, centuries long, paradigm like that. Imagination without complex, higher order, mental models is useless for discoveries like Einstein's.