Go backward to Operational InterpretationGo up to TopGo forward to Conjunctions |

*Proposition:* For every formula `A`, we have

~~AiffA.

*Proof*:
Let `A` be an arbitrary formula. We have to show that the meaning of
~~`A` is the same as the meaning of `A`, i.e., that they
have the same truth values.

A~ A~~ Afalsetruefalsetruefalsetrue

Since the last column coincides with the first column, we are done.

Author: Wolfgang Schreiner

Last Modification: October 6, 1999