Propositional Calculus Here is a definition of the formal system for propositional logic. 1. Symbols: A,B,C,D,...,Z(and optionally, allow primes, A0,A00, etc.) ∼,∨,∧,(,) and additionally the symbols ⇒ and ⇔, which are only shorthand for their equivalent forms (defined below). 2. Well Formed Formulas