criticism in his reviews of both editions of Principia Mathematica. Lewis's reputation as a promising young logician was soon assured.

Material implication (the rule of inference which claims that stating "P implies Q" is equivalent to stating "Q OR not P") allows a true consequent to follow from a false antecedent (so if P is not true still Q may be true since you only stated what a true P implies, but did not state what is implied if P is untrue). Lewis proposed to replace the usage of material implication during discussions involving