The structures of propositions stand to one another in internal relations.
5.21 We can bring out these internal relations in our manner of expression, by presenting a proposition as the result of an operation which produces it from other propositions (the bases of the operation).
5.22 The operation is the expression of a relation between the structures of its result and its bases.
5.23 (4) The operation is that which must happen to a proposition in order to make another out of it.
5.24 (2) An operation shows itself in a variable; it shows how we can proceed from one form of proposition to another.
It gives expression to the difference between the forms.
(And that which is common the the bases, and the result of an operation, is the bases themselves.)
5.25 (4) The occurrence of an operation does not characterize the sense of a proposition.
For an operation does not assert anything; only its result does, and this depends on the bases of the operation.
(Operation and function must not be confused with one another.)