If and Only If

If And Only If

↔ ⇔ ≡

Logical symbols
representing iff

In logic and related fields such as mathematics and philosophy, if and only if (shortened iff) is a biconditional logical connective between statements.

In that it is biconditional, the connective can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other, i.e., either both statements are true, or both are false. It is controversial whether the connective thus defined is properly rendered by the English "if and only if", with its pre-existing meaning. Of course, there is nothing to stop us stipulating that we may read this connective as "only if and if", although this may lead to confusion.

In writing, phrases commonly used, with debatable propriety, as alternatives to P "if and only if" Q include Q is necessary and sufficient for P, P is equivalent (or materially equivalent) to Q (compare material implication), P precisely if Q, P precisely (or exactly) when Q, P exactly in case Q, and P just in case Q. Many authors regard "iff" as unsuitable in formal writing; others use it freely.

In logic formulae, logical symbols are used instead of these phrases; see the discussion of notation.

Read more about If And Only If:  Definition, Distinction From "if" and "only If", More General Usage

Other articles related to "if and only if, and, if":

If And Only If - More General Usage
... It has the same meaning as above it is an abbreviation for if and only if, indicating that one statement is both necessary and sufficient for the other ... However, as noted above, if, rather than iff, is more often used in statements of definition.) The elements of X are all and only the elements of Y is used to mean "for any z in the ...

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