3.312 It is therefore presented by means of the general form of the
propositions that it characterizes. In fact, in this form the expression
will be constant and everything else variable.


3.313 Thus an expression is presented by means of a variable whose values
are the propositions that contain the expression. (In the limiting case the
variable becomes a constant, the expression becomes a proposition.) I call
such a variable a 'propositional variable'.


3.314 An expression has meaning only in a proposition. All variables can be
construed as propositional variables. (Even variable names.)


3.315 If we turn a constituent of a proposition into a variable, there is a
class of propositions all of which are values of the resulting variable
proposition. In general, 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.