Leibniz's Principle of Sufficient Reason

Comparison of Aquinas' treatment of infinite regress with Leibniz’s Principle of Sufficient Reason

One can see that Leibniz’s cosmological argument, although not as fully developed as Aquinas’, relies on the assumption that there cannot be an infinite regress of causes.

In an essay he wrote in 1697 called On the Ultimate Origination of Things, Leibniz writes,
“However far you go back to earlier states, you will never find in those states a full reason why there should be any world rather than none, and why it should be as it is. Therefore, even if you suppose the world eternal, as you will still be supposing nothing but a succession of states and will not in any of them find a sufficient reason... it is evident that the reason must be sought elsewhere.”

Leibniz argues that there must be a sufficient reason to explain the universe. He illustrates his point with an analogy like the one above:

“Let us suppose the book of the elements of geometry to have been eternal, one copy always to have been written down from an earlier one; it is evident that, even though a reason can be given for the present book of a past one, nevertheless out of any number of books taken in order going backwards we shall never come upon a full reason... why there are books at all, and why they were written in this manner.”

When one has a question about the origin of being, multiplying more and more being does not answer the question. Even by multiplying being to infinity, we still lack an answer. We need instead a sufficient reason or, as Aquinas calls it, an Unmoved Mover.