Previously, we used premises to show that a conclusion is true based on those premises. With tautologies, we have no premises at all!
- A tautology is a statement that is true regardless of the truth values of the simple sentences within it. For example, P v ~P is a tautology.
- Because tautologies start without premises, you have to use a proof by contradiction or conditional proof to show that it is true.
- From there, apply proof rules as usual.