If we change a constituent a of a proposition φ(a) into a variable, then there is a class
^
p
{(∃x).φ(x) = p}.
This class in general still depends upon what, by an arbitrary convention, we have 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.