We cannot define anything precisely! If we attempt to, we get into that paralysis of thought that comes to philosophers who sit opposite each other, one saying to the other, “You don’t know what you are talking about!”. The second one says, “What do you mean by know? What do you mean by talking? What do you mean by you?” and so on.

Richard Feynman (1918-1988) American physicist

The Feynman Lectures on Physics, Volume I, 8-2 “Motion” (20 Oct 1961)
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