necessary and sufficient conditions

Necessary and sufficient conditions are ways to describe relationships between ideas. A condition is *necessary* if it must be true for something else to happen; without it, the event can't occur. A condition is *sufficient* if, when it is true, it guarantees the event will happen. For example, having oxygen is necessary for fire—no fire without oxygen. However, oxygen alone isn't enough to start a fire; you also need something flammable. In summary, necessary conditions are required, and sufficient conditions guarantee an outcome if met.