General Theory -A. We Can Define a Tautologous Sentence as One That

Short Answer

General Theory
-a. We can define a tautologous sentence as one that is a substitution instance of some tautologous sentence form, and a contradictory sentence as one that is a substitution instance of some contradictory sentence form. Why can't we analogously define a contingent sentence as one that is a substitution instance of some contingent sentence form? (Defend your answer, including examples.)
b. We cannot define a contingent sentence as one that is a substitution instance of some contingent sentence form. But then how can we define that term?