Page:Russell, Whitehead - Principia Mathematica, vol. I, 1910.djvu/128

106 $$\scriptstyle{\vdash.p\or\sim\{\sim(\sim p)\}}$$

This proposition is a lemma for *2·14, which, with *2·12, constitutes the principle of double negation.

Dem.

$$\scriptstyle{\vdash.\sim(\sim p)\supset p}$$

Dem.

$$\scriptstyle{\vdash:\sim p\supset q.\supset.\sim q\supset p}$$

Dem.