5.5351 There are certain cases in which one is tempted to use expressions

of the form 'a = a' or 'p z p' and the like. 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.)

- -- Wittgenstein, Ludwig