The Principle of Sufficient Reason states that everything has an explanation. But different notions of explanation yield different versions of this principle. Here a version is formulated in terms of the notion of a "grounding" explanation. Its consequences are then explored, with particular emphasis on the fact that it implies necessitarianism, the view that every truth is necessarily true. Finally, the principle is defended from a number of objections, including objections to necessitarianism. The result is a defense of a "rationalist" metaphysics, one that constitutes an alternative to the contemporary dogmas that some aspects of the world are "metaphysically brute" and that the world could in so many ways have been different.Those who are ignorant of true causes, make complete confusion-think that trees might talk just as well as men-that men might be formed from stones as well as from seed . . .-B. Spinoza, Ethics, 1p8s2A metaphysical rationalist is someone who endorses the Principle of Sufficient Reason (PSR), the principle that everything has an explanation. The PSR can be understood in a number of different ways depending on the meaning of 'explanation' and the range of the quantifier 'everything', so metaphysical rationalists come in many different stripes. But every rationalist thinks that in some interesting sense there is an explanation (or "reason") for everything. Her rationalism is then qualified as being metaphysical to distinguish it from other rationalisms (such as epistemic rationalism, a thesis concerning the sources of justification) but this qualification can now be dropped for brevity.Although rationalism of one form or another was arguably embraced by such luminaries as Aquinas, Spinoza, and Leibniz (amongst others), it is fair to say that it has since largely fallen out of favor. Indeed my sense is that (with a few exceptions) contemporary philosophers tend to treat it as an antiquated view that need not be taken seriously today. 1 This may be the result of two factors. First, the PSR (in one form or another) is often thought to have consequences that might be thought problematic, including necessitarianism (the view that every truth is necessarily true), the existence of God, and indeed the existence of every conceivable being. 2 And second, there is a perceived lack of evidence in its favor. These two factors naturally lead to an attitude of (perhaps respectful) dismissal. C 2014 Wiley Periodicals, Inc.