Method of Truth Tables/Proof of Tautology/Examples
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Examples of Proof of Tautology
Peirce's Law
Consider the truth table for Peirce's Law:
- $P = \paren {\paren {p \implies q} \implies p} \implies p$
which is:
$\begin{array}{cc||ccccccc} p & q & ((p & \implies & q) & \implies & p) & \implies & p \\ \hline \F & \F & \F & \T & \F & \F & \F & \T & \F \\ \F & \T & \F & \T & \T & \F & \F & \T & \F \\ \T & \F & \T & \F & \F & \T & \T & \T & \T \\ \T & \T & \T & \T & \T & \T & \T & \T & \T \\ \end{array}$
The main connective of $P$ is the rightmost instance of $\implies$.
The column beneath that connective is all $\T$, so $\paren {\paren {p \implies q} \implies p} \implies p$ is a tautology.
$\blacksquare$