The principle of plenitude asserts that the universe contains all possible forms of existence.
Arthur Lovejoy, a historian of ideas, was the first to trace the history of this philosophically important principle explicitly.
Lovejoy distinguishes two versions of the principle: a static version, in which the universe displays a constant fullness and diversity, and a temporalized version, in which fullness and diversity gradually increase over time.
Lovejoy traces the principle of plenitude to the writings of Plato, finding in the Timaeus an insistence on "the necessarily complete translation of all the ideal possibilities into actuality".
[1] By contrast, he takes Aristotle to reject the principle in his Metaphysics, when he writes that "it is not necessary that everything that is possible should exist in actuality".