What is the nature and structure of reality? Aristotle famously describes metaphysics as the study of "being qua being" and Descartes places metaphysics as “the root” of the tree of Philosophy.” Metaphysicians want to know what the world is like, and tend to ask questions about what sorts of things exist (e.g. are there numbers?), as well as what sorts of things are fundamental (e.g. is everything made of simple elements?). Among the more central subtopics of metaphysics are the topics of the nomological (including laws of nature and causation), the modal (including the nature of possibility), and the fundamental (including the relation between the fundamental and the derivative). Other central topics include time, the structure of objects (including human persons), and the nature of properties. There is also an ongoing discussion of meta-metaphysics, concerning the nature and viability of metaphysics itself.