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.