If we change a constituent
a of a proposition
φ(a) into a variable, then
there is a class
This class in general still depends upon what, by an
arbitrary
convention, we mean by “
φ(x)”.
But if we change into variables all those symbols whose significance was
arbitrarily determined, there is still such a class.
But this is now not dependent upon any convention, but only upon the
nature of the symbol “
φ(x)”.
It corresponds to a logical type.