principle of plenitude the principle that every genuine possibility is realized or actualized. This principle of the ‘fullness of being’ was named by A. O. Lovejoy, who showed that it was commonly assumed throughout the history of Western science and philosophy, from Plato to Plotinus (who associated it with inexhaustible divine productivity), through Augustine and other medieval philosophers, to the modern rationalists (Spinoza and Leibniz) and the Enlightenment. Lovejoy connected plenitude to the great chain of being, the idea that the universe is a hierarchy of beings in which every possible form is actualized. In the eighteenth century, the principle was ‘temporalized’: every possible form of creature would be realized – not necessarily at all times – but at some stage ‘in the fullness of time.’ A clue about the significance of plenitude lies in its connection to the principle of sufficient reason (everything has a sufficient reason [cause or explanation] for being or not being). Plenitude says that if there is no sufficient reason for something’s not being (i.e., if it is genuinely possible), then it exists – which is logically equivalent to the negative version of sufficient reason: if something does not exist, then there is a sufficient reason for its not being. R.H.K.