A little background: in his Lectures on Gravitation, Feynman takes a typically idiosyncratic approach to General Relativity, deriving it in a non-geometrical formulation as the natural consequence of a demand to produce a massless spin-2 field that can be coupled to the energy-momentum tensor. (This is absolutely NOT where Einstein started!). Nonetheless, in these lectures and elsewhere, he makes several comments about his respect for Einstein’s brilliance in coming up with GR.
In particular, he makes the following remark, which may be the quote that the question is referring to (N.B., for non-physicists, the “Lagrangian” referred to below can be thought of as an expression summing up the dynamics of the theory — it’s not the whole theory but it is its essence):
Einstein himself, of course, arrived at the same Lagrangian but without the help of a developed field theory, and I must admit that I have no idea how he guessed the final result. We have had troubles enough arriving at the theory – but I feel as though he had done it while swimming underwater, blindfolded, and with his hands tied behind his back!”
#theories of relativity