Mathematical Logic Proof of Syllogism
Law 1
The following hypothetical proposition is called a syllogism. $$ ( p \to q ) \land ( q \to r ) \implies p \to r $$
Explanation
There is hardly anyone who does not know about the syllogism, and it is believed there’s no need for explicit explanation. Except in ancient philosophical debates, it’s rare to use the phrase ‘by syllogism.’ That’s because it is a familiar method of reasoning and a universally valid principle to all of us.
However, few people would have thought that syllogism needs to be proven or could be proven. Let’s prove the syllogism using the logical symbols used in mathematics.
Proof
Logical equivalence of the conditional: $$ p \to q \equiv \left( \lnot p \lor q \right) \qquad \cdots \star $$
- [2] Simplification law: $$ p \land q \implies p \\ p \land q \implies q $$
- [7] Association law: $$ (p \land q) \land r \iff p \land (q \land r) \\ (p \lor q) \lor r \iff p \lor (q \lor r) $$
- [8] Distribution law: $$ p \land (q \lor r) \iff (p \land q) \lor (p \land r) \\ p \lor (q \land r) \iff (p \lor q) \land (p \lor r) $$
Therefore, because of $( p \to q ) \iff (\lnot p \lor q)$ $$ \begin{align*} & ( p \to q ) \land ( q \to r ) \\ \iff & (\lnot p \lor q) \land (\lnot q \lor r) & \because \star \\ \iff & (\lnot p \land \lnot q) \lor (\lnot p \land r) \lor (q \land \lnot q) \lor (q \land r) & \because [7], [8] \\ \implies& \lnot p \lor \lnot p \lor c \lor r & \because [2] \\ \implies& \lnot p \lor r \\ \implies& p \to r & \because \star \end{align*} $$ Here, $c$ represents a contradiction.
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이흥천 역, You-Feng Lin. (2011). 집합론(Set Theory: An Intuitive Approach): p31. ↩︎