Outlines a conceptual argument against the Principle of Sufficient reason. The argument is presented in detail in earlier work, and is based on deductive inferences from PSR's own concept of explanation. The argument shows that not everything can have an explanation of the sort claimed by PSR. So far from being a presupposition of reason itself, as some think, PSR can be refuted by reason, arguing only from PSR's own concept of explanation. Hence PSR cannot be used to argue that there must be some explanation or reason for existence, invisible at least to science, or that because we do not or cannot know the explanation, there must be irreducible mystery about why there is anything at all rather than nothing, including why there was a Big Bang in the first place.