Keynes quotes from BR's "second" letter in his letter to BR (see record 77716).

"It seems to me absolutely evident that if nothing whatever is given about x except that it satisfies Qx, then the probability of its satisfying Qx cannot be dependant upon its being the particular x that it is. To suppose otherwise seems to me only possible by slipping in surreptitiously what you know about x out and above Qx."