From the dedication of C. S. Lewis’ A Preface to Paradise Lost:
Apparently the door of the prison was really unlocked all the time; but it was only you who thought of trying the handle.
In context he was saying that Charles Williams had recovered a clear understanding of Milton that had been obfuscated for over a century.
That line made me think of a quote from Alexander Grothendieck (that I haven’t been able to find this morning) to the effect that progress in mathematics has often waited for someone to be bold enough to ask a simple question or introduce a simple concept.
Update: Thanks to Roland Elliot for providing the quote I was missing. Ronald Brown says that Grothendieck said in a letter to him in 1982:
The introduction of the cipher 0 or the group concept was general nonsense too, and mathematics was more or less stagnating for thousands of years because nobody was around to take such childish steps …