Nihil est sine ratione, "nothing is without reason." For everything that is, there is an answer to the question why it is just so and not otherwise-and, ultimately, there is a reason (ratio) grounding the fact that there is something rather than nothing. Reality as a whole is thus "rational" or "reasonable," in the sense that everything is based on something and has a "why" or a "how come" that grounds it, rendering it comprehensible and meaningful and thus letting it be part of meaningful reality. Nothing is without something that lets it be what it is. This is the famous "principle of reason" (principium rationis)-in a slightly different formulation, the "principle of the restoration of sufficient reason" (principium reddendae rationis sufficientis)-which was first explicitly formulated in these words by G. W. Leibniz, although not as a doctrine of his own but as a fundamental and generally accepted philosophical principle. There is in Nature a reason [ratio] why something should exist rather than nothing. This is a consequence of the great principle that nothing comes to be without reason, just as there also must be a reason why this exists, rather than something else.1 Indeed, in one form or another, the principle of reason has been an integral part of Occidental metaphysics ever since antiquity. The central issue of Aristotle's Metaphysics-the question which Heidegger names the leading question (Leitfrage) of Occidental metaphysics as a whole-concerns that-which-is, beings.2 What is a being as such? What is a being insofar as it is a being? Or, as Aristotle reformulates the question, what is the being-ness (ousia), the fundamental being-character, of beings as such? For Aristotle, the first essential characteristic of being-ness is fundamentality, foundationality.3 What truly is must be a hypokeimenon (in Latin substantia, "substance," or subiectum, "subject")-literally, something that "lies beneath" as an ontological basis or foundation, letting other, dependent beings be while itself remaining ontologically independent. The most being of all beings is that on which all other beings are dependent but which is itself absolutely independent of anything beyond itself. The ontological investigation of Books Zeta, Eta, and Theta of Aristotle's Metaphysics thus finds its culmination in the theology of Book Lambda-a discussion of God as the absolute, the most fundamental and necessary being-ness and principle. The Aristotelian notion of God as absolute self-awareness, as perfect being in-itself and for-itself, was then taken over by medieval Scholasticism-for St. Thomas, God as the uncreated creator and the ultimate cause of things is not only the most being of beings (maxime ens) but subsistence and permanence, that is, being-ness, as such (ipsum esse subsistens).4 Heidegger's famous genealogy of modernity shows how, since Descartes, the self-conscious I, now interpreted with regard to the absolute and immediate self-certainty of the cogito, gradually replaces God as the fundamental subject of reality. As modern metaphysics unfolds, human subjectivity as the basis of meaningfulness becomes more and more absolute, and self-sufficient; this "subjectivization" culminates in Nietzsche's idea of the "superhuman" subject who no longer simply apprehends given objectivity but instead gives itself its own "truths" as fuel for its essence, i.e., the ceaseless will to self-enhancement.5 In asking its leading question concerning the being-ness of beings, the metaphysical tradition has, according to Heidegger, constantly sought a supreme, ideal form of being-ness which all beings could be referred back to and founded upon. Yet from Plato to Nietzsche, metaphysics has been unable to radically pose what Heidegger calls the "basic question" or "fundamental question" (Grundfrage) of philosophy;6 it has never really inquired into the origin of its own "rationality." What are the experience of ideal being-ness as something foundational and the subsequent metaphysical demand for foundations in themselves based on? …