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 …
You mean this one?
“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…”
I find it amazing that it took me so long to hear about grothendieck. Wonder why, since he clearly rewrote mathematics to a great extent. Mathematicians complain that people think their field is static, but how is it that they don’t deploy the story of this remarkable man to hook and teach us, like how physicists (mostly Feynman) used the antics of Feynman.