
Conversation of 1930, in Personal Recollections (1981) by Rush Rhees, Ch. 6
Variant: Philosophy is like trying to open a safe with a combination lock: each little adjustment of the dials seems to achieve nothing, only when everything is in place does the door open.
Source: 1930s-1951, Philosophical Occasions 1912-1951 (1993), Ch. 9 : Philosophy, p. 175