Principle that if the universe is to be as perfect as possible it must be as full as possible, in the sense that it contains as many kinds of things as it possibly could contain.
The world of nature must be as rich as possible. This is connected with the idea, used by St Anselm (1033-1109) in his ontological argument for God's existence, that existence is a perfection.
Another version of the principle refers to events rather than to kinds of object. It says that there can be no possibilities that remain as possibilities (and are not foreclosed) but are unrealized throughout eternity; in this form, which goes back at least to Aristotle (384-322 BC), the principle is given some credibility by PROBABILITY THEORY: the probability that the proverbial monkey at a typewriter will type a Shakespeare sonnet straight off may be minute, but if he remains typing for long enough the probability increases indefinitely and it becomes increasingly surprising if he does not type one.
A O Lovejoy, The Great Chain of Being (1936)