Reading some old logic texts (written around 1930) I noticed that these texts make no difference between propositional variables and terms.

They do make difference between identity and truthvalue but for the rest they are just treated the same.

(p = q) -> ( p <-> q) ( if p is identical to p then p and q have the same truthvalue, equivalent) is a wellformed formula.

When did logicians start to differentiate between terms and propositional variables?

and more important why?