# Necessity and sufficiency

 "Gaut does not acknowledge other versions of the cluster theory, but there have been several. E. J. Bond (1975) observes that a set of conditions may be sufficient for something's being art though no single member of the set is either necessary or sufficient. Milton H. Snoeyenbos (1978) argues that "art" may be used on the basis of a disjunctive set of merely sufficient conditions. Ellen Dissanayake observes that 'one method of definition has been to compile a number of attributes of art.'" --"The Cluster Theory of Art" (2004) by Stephen Davies

In logic, necessity and sufficiency refer to the implicational relationships between statements. The assertion that one statement is a necessary and sufficient condition of another means that the former statement is true if and only if the latter is true.

## Relationship between necessity and sufficiency

A condition can be either necessary or sufficient without being the other. For instance, being a mammal (N) is necessary but not sufficient to being human (S), and that a number [itex]x[/itex] is rational (S) is sufficient but not necessary to [itex]x[/itex] being a real number (N) (since there are real numbers that are not rational).

A condition can be both necessary and sufficient. For example, at present, "today is the Fourth of July" is a necessary and sufficient condition for "today is Independence Day in the United States". Similarly, a necessary and sufficient condition for invertibility of a matrix M is that M has a nonzero determinant.

Mathematically speaking, necessity and sufficiency are dual to one another. For any statements S and N, the assertion that "N is necessary for S" is equivalent to the assertion that "S is sufficient for N". Another facet of this duality is that, as illustrated above, conjunctions (using "and") of necessary conditions may achieve sufficiency, while disjunctions (using "or") of sufficient conditions may achieve necessity. For a third facet, identify every mathematical predicate N with the set T(N) of objects, events, or statements for which N holds true; then asserting the necessity of N for S is equivalent to claiming that T(N) is a superset of T(S), while asserting the sufficiency of S for N is equivalent to claiming that T(S) is a subset of T(N).

### Argument forms involving necessary and sufficient conditions

#### Invalid forms of argument (i.e. fallacies)

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