Definition $2\mathrm{.}1\mathrm{.}34.$ A tautology is a propositional formula which is true regardless of the truth values assigned to its variables.

Example $2\mathrm{.}1\mathrm{.}35.$ Let $p$ and $q$ be propositions. We'll prove that

$)=>(p$

is a tautology by looking at its truth table: