There's something I don't understand in my course on propositional logic.
In the case of x being a variable, the definition of its dual is x* = x. Right.
However, further in the course, there's a corollary stating : In propositional logic, if a formula is valid, its dual is contradictory.
How can this be true if x* = x ?
Does it even make sense to talk about the truth value of a variable ?
I'm a little bit confused...