(related to Lemma: Denying the Antecedent of an Implication)
We want to prove that the mixing-up the sufficient and necessary conditions is a fallacy.
* The fallacy can be formulated in propositional logic as (p\Rightarrow q)\wedge \neg p\Rightarrow \neg q.
* The definitions of the implication "$\Rightarrow$
" and the negation "\neg" give us the following truth table of the function:
[[p]]_I | [[q]]_I | [[p\Rightarrow q]]_I | [[\neg p]]_I | [[\neg q]]_I |
---|---|---|---|---|
0 | 0 | 1 | 1 | 1 |
0 | 1 | 1 | 1 | 0 |
1 | 0 | 0 | 0 | 1 |
1 | 1 | 1 | 0 | 0 |