Full Idea
Necessary and sufficient conditions are usually expressed by "if and only if" (abbr. "iff"), where "if" is the sufficient condition, and "only if" is the necessary condition.
Gist of Idea
In "if and only if" (iff), "if" expresses the sufficient condition, and "only if" the necessary condition
Source
Pascal Engel (Truth [2002], §1.1)
Book Reference
Engel,Pascal: 'Truth' [Acumen 2002], p.14
A Reaction
'I take my umbrella if and only if it is raining' (oh, and if I'm still alive). There may be other necessary conditions than the one specified. Oh, and I take it if my wife slips it into my car…
Related Idea
Idea 3891 If p entails q, then p is sufficient for q, and q is necessary for p [Scruton]