|Rules of inference|
|Rules of replacement|
Conjunction introduction (often abbreviated simply as conjunction and also called and introduction) is a valid rule of inference of propositional logic. The rule makes it possible to introduce a conjunction into a logical proof. It is the inference that if the proposition p is true, and proposition q is true, then the logical conjunction of the two propositions p and q is true. For example, if it's true that it's coming, and it's true that I'm inside, then it's true that "it's coming and I'm inside". The rule can be stated:
where the rule is that wherever an instance of "" and "" appear on lines of a proof, a "" can be placed on a subsequent line.
The conjunction introduction rule may be written in sequent notation:
where and are propositions expressed in some formal system.