Some people claim that the term 'metaphysics' comes simply from the title of a work of Aristole. His book subsequent to the physics 'the metaphysics' simply meaning beyond, after, or above the physics. It has however also come to mean foundational or 'first philosophy', which in the case of Aristotle dealt with the central question of being - almost identical with the idea of substance. In Descartes's Principles of Philosophy'he structures the whole of philosophy like a tree, where the roots are metaphysics, the trunk represents physics with its branches representing the other sciences. For Liebniz too, the principles of mechanics etc flow from metaphyscial principles. For Heidegger, 'metaphysics is 'onto-theology' or better 'onto-theo-logic', concerning questions about the grounds of beings as such and as a whole. Heidegger proposes to 'step back' out of metaphysics into the essential nature of metaphysics, looking towards an unthought in our thinking about being.
Metaphysics thinks of beings as such, that is, in general. Metaphysics thinks of beings as such, as a whole. Metaphysics thinks of the Being of beings both in in the ground-giving unity of what is most general, what is indifferently valid everywhere, and also in the unit of the all that accounts for the ground, that is, of the All-Highest. The Being of beings is thus thought of in advance as the grounding ground. Therefore all metaphysics is at bottom, and from the ground up, what grounds, what gives account of the ground, what is called to account by the ground, and finally what calls the ground to account. (Heidegger (2002) p. 58)