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Full Idea
My knowing that I had a hidden birth-mark would not entitle me to infer with any great degree of confidence that the same was true of everybody else.
Gist of Idea
You can't infer that because you have a hidden birth-mark, everybody else does
Source
A.J. Ayer (The Central Questions of Philosophy [1973], §VI.E)
Book Ref
Ayer,A.J.: 'The Central Questions of Philosophy' [Penguin 1976], p.134
A Reaction
This is the notorious 'induction from a single case' which was used by Mill to prove that other minds exist. It is a very nice illustration of the weakness of arguments from analogy. Probably analogy on its own is useless, but is a key part of induction.