al, this class too will be dependent on
the meaning that our arbitrary conventions have given to parts of the
original proposition. But if all the signs in it that have arbitrarily
determined meanings are turned into variables, we shall still get
a class of this kind. This one, however, is not dependent on any
convention, but solely on the nature of the pro position. It corresponds
to a logical form--a logical prototype.

3.316 What values a propositional variable may take is something that is
stipulated. The stipulation of values is the variable.

3.317 To stipulate values for a propositional variable is to give
the propositions whose common characteristic the variable is. The
stipulation is a description of those propositions. The stipulation will
therefore be concerned only with symbols, not with their meaning. And
the only thing essential to the stipulation is that it is merely a
description of symbols and states nothing about what is signified. How
the description of the propositions is produced is not essential.

3.318 Like Frege and Russell I construe a proposition as a function of
the expressions contained in it.

3.32 A sign is what can be perceived of a symbol.

3.321 So one and the same sign (written or spoken, etc.) can be common
to two different symbols--in which case they will signify in different
ways.

3.322 Our use of the same sign to signify two different objects can
never indicate a common characteristic of the two, if we use it with two
different modes of signification. For the sign, of course, is arbitrary.
So we could choose two different signs instead, and then what would be
left in common on the signifying side?

3.323 In everyday language it very frequently happens that the same
word has different modes of signification--and so belongs to different
symbols--or that two words that have different modes of signification
are employed in propositions in what is superficially the same way. Thus
the word 'is' figures as the copula, as a sign for identity, and as an
expression for existence; 'exist' figures as an intransitive verb like
'go', and 'identical' as an adjective; we speak of something, but also
of something's happening. (In the proposition, 'Green is green'--where
the first word is the proper name of a person and the last an
adjective--these words do not merely have different meanings: they are
different symbols.)

3.324 In this way the most fundamental confusions are