necessity

Necessity refers to the state or quality of being required or indispensable for a purpose or outcome. In ordinary usage, it denotes things that must be obtained or actions that must be taken to satisfy basic needs, ensure safety, or achieve a goal. Essentials such as food, water, shelter, and lawful conduct are commonly described as necessities.

In philosophy, necessity is a modal concept distinguishing what must be the case from what could be the case. Logical or analytic necessity holds in virtue of meaning or logic; statements such as “2+2=4” or “All bachelors are unmarried” are true in all possible worlds. Contingent truths depend on facts about the world and could have been otherwise. Metaphysical necessity is the idea that certain truths are true in every possible world because of the true nature of things, though this is a topic of ongoing debate.

In law and ethics, the doctrine of necessity refers to a justification for action that would otherwise be illegal or wrongful, if it is taken to prevent imminent and grave harm. For example, breaking a seal to save a life may be permitted under necessity.

In broader discourse, necessity is often used synonymously with essential or mandatory, indicating properties, conditions, or policies deemed indispensable for functioning, safety, or public welfare. The concept intersects with discussions of ontology, epistemology, and political philosophy, and its precise meaning varies with context and discipline.