Quantum mechanics… developed through some rather messy, complicated processes stimulated by experiment. While it's a very rich and wonderful theory, it doesn't quite have the conceptual foundation of general relativity.
String theory is extremely attractive because gravity is forced upon us.
No one invented it on purpose, it was invented in a lucky accident. …By rights, string theory shouldn't have been invented until our knowledge of some of the areas that are prerequisite… had developed to the point that it was possible for us to have the right concept of what it is all about.
String theory at its finest is, or should be, a new branch of geometry. …I, myself, believe rather strongly that the proper setting for string theory will prove to be a suitable elaboration of the geometrical ideas upon which Einstein based general relativity.