Principle of Sufficient Reason: The claim that there must be an explanation for every positive fact, some reason why that fact obtains rather than not obtaining. This principle is generally attributed to Gottfried Leibniz, for whom it took the form of the assumption that God has a sufficient reason for every choice he has made. The principle, or some variation on it, often plays a key role in cosmological arguments for the existence of the finite universe. Those who deny the principle of sufficient reason are committed to the claim that some facts obtain for no reason, and thus that there is a surd (nonrational) element to the universe.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), p. 112.
What was last year's post? See here: James White's debate with Bart Ehrman.