In fact, this happens when one
wants to talk about prototypes, e.g. about proposition, thing, etc. Thus in
Russell's Principles of Mathematics 'p is a proposition'--which is nonsense-
-was given the symbolic rendering 'p z p' and placed as an hypothesis in
front of certain propositions in order to exclude from their argument-
places everything but propositions. (It is nonsense to place the hypothesis
'p z p' in front of a proposition, in order to ensure that its arguments
shall have the right form, if only because with a non-proposition as
argument the hypothesis becomes not false but nonsensical, and because
arguments of the wrong kind make the proposition itself nonsensical, so
that it preserves itself from wrong arguments just as well, or as badly, as
the hypothesis without sense that was appended for that purpose.)
5.5352 In the same way people have wanted to express, 'There are no things
', by writing 'P(dx) . x = x'. But even if this were a proposition, would
it not be equally true if in fact 'there were things' but they were not
identical with themselves?
5.54 In the general propositional form propositions occur in other
propositions only as bases of truth-operations.
5.541 At first sight it looks as if it were also possible for one
proposition to occur in another in a different way. Particularly with
certain forms of proposition in psychology, such as 'A believes that p is
the case' and A has the thought p', etc.
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