Peirce's Law

Theorem
Peirce's law is a formula in propositional calculus that is commonly expressed in the following form:


 * $$((p \implies q) \implies p) \vdash p$$

or, which is equivalent:
 * $$((p \implies q) \implies p) \implies p$$

Peirce's law holds in classical propositional calculus, but not in intuitionistic propositional calculus. The precise axiom system that one chooses for classical propositional calculus determines whether Peirce's law is taken as an axiom or proven as a theorem.

Under the existential interpretation of logical graphs, Peirce's law is represented by means of the following formal equivalence or logical equation:

Strong Form of Peirce's Law
The logical implication does in fact go both ways:
 * $$((p \implies q) \implies p) \dashv \vdash p$$

or equivalently:
 * $$((p \implies q) \implies p) \iff p$$

Representing propositions as logical graphs under the existential interpretation, the strong form of Peirce's law is expressed by the following equation:

Proof by Logical Graphs
First we show $$((p \implies q) \implies p) \implies p$$:

Using the axioms and theorems listed in the entry for logical graphs, Peirce's law may be proved in the following manner.

Similarly, the strong form may be proved in the following manner:

Proof by Natural deduction
By the Tableau method:

Now from Implication Properties, we have $$p \vdash r \implies p$$ which is equivalent to $$p \implies (r \implies p)$$.

Now we put a substitution instance of $$p \implies q$$ for $$r$$, and:
 * $$p \implies ((p \implies q) \implies p)$$

follows immediately.

Proof by Truth Table
We can directly prove $$((p \implies q) \implies p) \dashv \vdash p$$.

We apply the Method of Truth Tables to the proposition.

As can be seen by inspection, the truth values under the main connectives match for all models.

$$\begin{array}{|ccccc||c|}\hline ((p & \implies & q) & \implies & p) & p \\ \hline F & T & F & F & F & F \\ F & T & T & F & F & F \\ T & F & F & T & T & T \\ T & T & T & T & T & T \\ \hline \end{array}$$

Comment
A non-obvious result that has the same strength as the Law of the Excluded Middle.

History
Here is Peirce's own statement and proof of the law:

A fifth icon is required for the principle of excluded middle and other propositions connected with it. One of the simplest formulae of this kind is:

$$\{ (x \prec y) \prec x \} \prec x$$.

This is hardly axiomatical. That it is true appears as follows. It can only be false by the final consequent $$x\!$$ being false while its antecedent $$(x \prec y) \prec x$$ is true. If this is true, either its consequent, $$x\!$$, is true, when the whole formula would be true, or its antecedent $$x \prec y$$ is false. But in the last case the antecedent of $$x \prec y$$, that is $$x\!$$, must be true. (Peirce, CP 3.384).

Peirce goes on to point out an immediate application of the law:

From the formula just given, we at once get:

$$\{ (x \prec y) \prec a \} \prec x$$,

where the $$a\!$$ is used in such a sense that $$(x \prec y) \prec a$$ means that from $$(x \prec y)$$ every proposition follows. With that understanding, the formula states the principle of excluded middle, that from the falsity of the denial of $$x\!$$ follows the truth of $$x\!$$. (Peirce, CP 3.384).

Note. The above transcription uses the "precedes sign" ($$\prec$$) for the "sign of illation" that Peirce customarily wrote as a cursive symbol somewhat like a gamma ($$\gamma\!$$) turned on its side or else typed as a bigram consisting of a dash and a "less than" sign.