Peirce's law

Peirce's law in logic is named after the philosopher and logician Charles Sanders Peirce. It was taken as an axiom in his first axiomatisation of propositional logic. The axiom can be used as an alternative to the law of excluded middle.

In propositional calculus, Peirce's law says that ((P→Q)→P)→P. Written out, this says that P must be true if you can show that P implying Q forces P to be true.

Peirce's law does not hold in intuitionistic logic or intermediate logics and cannot be deduced from the deduction theorem alone.

Under the Curry-Howard isomorphism, Peirce's law is the type of continuation operators.

History
Here is Peirce's own statement 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:


 * 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 —< y) —< x is true.  If this is true, either its consequent, x, is true, when the whole formula would be true, or its antecedent x —< y is false.  But in the last case the antecedent of x —< y, that is x, must be true.  (Peirce, the Collected Papers 3.384).

Peirce goes on to point out an immediate application of the law:
 * From the formula just given, we at once get:


 * where the a is used in such a sense that (x —< y) —< a means that from (x —< 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, the Collected Papers 3.384).

Other proofs of Peirce's law
Showing Peirce's Law applies does not mean that P→Q or Q is true, we have that P is true but only (P→Q)→P, not P→(P→Q) (see affirming the consequent).

simple proof: $$ (p \rightarrow q) \rightarrow p \Rightarrow \overline{p \rightarrow q} \or p \Rightarrow \overline{\overline p \or q} \or p \Rightarrow (p \and \overline q) \or p \Rightarrow (p \and \overline q) \or (p \and 1) \Rightarrow p \and (\overline q \or 1) \Rightarrow p \and 1 \Rightarrow p $$

Using Peirce's law with the deduction theorem
Peirce's law allow's one to enhance the technique of using the deduction theorem to prove theorems. Suppose one is given a set of premises &Gamma; and one wants to deduce a proposition Z from them. With Peirce's law, one can add (at no cost) additional premises of the form Z→P to &Gamma;. For example, suppose we are given P→Z and (P→Q)→Z and we wish to deduce Z so that we can use the deduction theorem to conclude that (P→Z)→(((P→Q)→Z)→Z) is a theorem. Then we can add another premise Z→Q. From that and P→Z, we get P→Q. Then we apply modus ponens with (P→Q)→Z as the major premise to get Z. Applying the deduction theorem, we get that (Z→Q)→Z follows from the original premises. Then we use Peirce's law in the form ((Z→Q)→Z)→Z and modus ponens to derive Z from the original premises. Then we can finish off proving the theorem as we originally intended.
 * P→Z 1. hypothesis
 * (P→Q)→Z 2. hypothesis
 * Z→Q 3. hypothesis
 * P 4. hypothesis
 * Z 5. modus ponens using steps 4 and 1
 * Q 6. modus ponens using steps 5 and 3
 * P→Q 7. deduction from 4 to 6
 * Z 8. modus ponens using steps 7 and 2
 * (Z→Q)→Z 9. deduction from 3 to 8
 * ((Z→Q)→Z)→Z 10. Peirce's law
 * Z 11. modus ponens using steps 9 and 10
 * ((P→Q)→Z)→Z 12. deduction from 2 to 11
 * (P→Z)→((P→Q)→Z)→Z) 13. deduction from 1 to 12 QED

Completeness of the implicational propositional calculus
One reason that Peirce's law is important is that it can substitute for the law of excluded middle in the logic which only uses implication (see implicational propositional calculus). The sentences which can be deduced from the axiom schemas: (where P,Q,R contain only "→" as a connective) are all the tautologies which use only "→" as a connective.
 * P→(Q→P)
 * (P→(Q→R))→((P→Q)→(P→R))
 * ((P→Q)→P)→P
 * from P and P→Q infer Q