. A necessary condition for something is one without which the thing does not exist or occur. The presence of oxygen is a necessary condition for human life. A sufficient condition for something is one given which the thing does exist or occur. Prolonged absence of oxygen is a sufficient condition for human death. Necessary or sufficient conditions need not precede what they are conditions of.
The existence of humans is a sufficient condition for the presence of oxygen. As this last example shows, a sufficient condition may be causally connected with what it is a condition of, without being a cause of it. But necessary or sufficient conditions do not have to be causal: fine weather is a necessary condition of my going out today—because I have so decided. They can also be logical (being an equilateral Euclidean triangle is both necessary and sufficient for being an equiangular Euclidean triangle), or just accidental: if all dodos were eaten, however accidentally, being eaten is a necessary (though not causally necessary) condition for being a dodo. But ‘necessary’ and ‘sufficient’ are often tacitly limited to the non-accidental. ‘Sufficient conditions’ (plural) may refer to conditions jointly sufficient for something, or to conditions each of which is itself sufficient.
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