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The truth is that being is a useless word, and that the kind of things to which this useless word is applied by those who mistakenly use it are just as apt to be many as to be one. One is a characteristic, not of things, but of certain propositional functions, namely, of those propositional functions which have the following property: there is an x which makes the function true and which is such that, if y makes the function true, y is identical with x. This is the definition of unitary functions, and the number 1 is the property of being unitary which certain functions possess.
Source: My Philosophical Development, 1959, by Bertrand Russell
More info.: https://russell-j.com/beginner/BR_MPD_06-050.HTM