Biconditional introduction: Difference between revisions

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{{Short description|Inference in propositional logic}}

{{Infobox mathematical statement

| name = Biconditional introduction

| type = [[Rule of inference]]

| field = [[Propositional calculus]]

| statement = If <math>P \to Q</math> is true, and if <math>Q \to P</math> is true, then one may infer that <math>P \leftrightarrow Q</math> is true.

| symbolic statement = <math>\frac{P \to Q, Q \to P}{\therefore P \leftrightarrow Q}</math>

}}

{{Transformation rules}}

In [[propositional calculus|propositional logic]], '''biconditional introduction'''<ref>Hurley</ref><ref>Moore and Parker</ref><ref>Copi and Cohen</ref> is a [[validityValidity (logic)|valid]] [[rule of inference]]. It allows for one to [[inference|infer]] a [[Logical biconditional|biconditional]] from two [[Material conditional|conditional statements]]. The rule makes it possible to introduce a biconditional statement into a [[formal proof|logical proof]]. If <math>P \to Q</math> is true, and if <math>Q \to P</math> is true, then one may infer that <math>P \leftrightarrow Q</math> is true. For example, from the statements "if I'm breathing, then I'm alive" and "if I'm alive, then I'm breathing", it can be inferred that "I'm breathing [[if and only if]] I'm alive". Biconditional introduction is the [[Converse (logic)|converse]] of [[biconditional elimination]]. The rule can be stated formally as:

:<math>\frac{P \to Q, Q \to P}{\therefore P \leftrightarrow Q}</math>

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or as the statement of a truth-functional [[Tautology (logic)|tautology]] or [[theorem]] of propositional logic:

:<math>((P \to Q) \andland (Q \to P)) \to (P \leftrightarrow Q)</math>

where <math>P</math>, and <math>Q</math> are propositions expressed in some [[formal system]].