Possibility is a central concept in metaphysics concerned with what could exist, happen, or be true under some set of conditions. Philosophical inquiry into possibility asks how unrealized alternatives relate to actuality, whether possible states of affairs have any kind of status beyond imagination, and how possibility differs from necessity, contingency, causation, identity, and existence. Questions about possibility are especially important when considering whether reality could have been otherwise and how alternative outcomes relate to the structure of the world.