Philosophical problems can be compared to locks on safes, which can be opened by dialing a certain word or number, so that no force can open the door until just this word has been hit upon, and once it is hit upon any child can open it.
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Notes from Wikiquote
Conversation, c. 1930, in Personal Recollections (1981) by Rush Rhees, Ch. 6
Variants
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.