3.311 An expression presupposes the forms of all propositions in which it can occur. It is the common characteristic feature of a class of propositions.
An expression (or symbol) is thus something like a meaning. In the way that a proposition can be thought of as what various sentences with the same meaning have in common, so too an expression is what various propositions with (or containing) the same meaning have in common. So do we need the concept of an expression? Do we need the concept of a proposition?