necessity and sufficiency

Necessity and sufficiency are concepts used to describe the relationship between conditions and outcomes. A condition is **necessary** if the outcome can't happen without it—think of it as an essential ingredient. For example, oxygen is necessary for fire; without oxygen, fire can't exist. A condition is **sufficient** if, when it occurs, the outcome will always happen—it's enough on its own to produce that result. For instance, lighting a match in dry grass is sufficient to start a fire. Sometimes, a condition can be necessary but not sufficient, or sufficient but not necessary, depending on the context.