7.1.2 False elimination

A funny one:

$$\begin{fitch*} \par n & \square \\ \par \hline \par & A & E$\square$\ n \par \end{fitch*}$$

Explanation: if we achieved the conclusion that $\square$ is true, then we have already achieved a state where we can invent anything and affirm that it's true; at least, as true as the idea of $\square$ (false) being true.

This rule is called ex falso quodlibet sequitur, something like ``from false can follow anything''.