if we take Φ to be a propositional formula we can also take ¬ {\displaystyle \neg } Φ to be a formula; if we take Φ and Ψ to be a propositional formulas we can also take (Φ ∧ {\displaystyle \wedge } Ψ), (Φ → {\displaystyle \to } Ψ), (Φ ∨ {\displaystyle \lor } Ψ) and (Φ ↔ {\displaystyle \leftrightarrow } Ψ) to also be formulas.