This volume addresses the subject of categories: What are they? How are they used in speaking and thinking? What role do they play in our moral deliberations? Why are there different sorts of categories? And are categories independent of our thinking and speaking, giving objective form to the world we aim to think and speak about? These and other questions concerning categories have been part of philosophy from the very beginning, and they raise foundational issues in metaphysics, epistemology, ethics, and other branches of philosophy. Yet pursuing answers to these questions has proven difficult, because investigations into categories push us to the very limits of what we can know. The essays in this volume, written by a mix of well-established and younger philosophers, bridge divides between historical and systematic approaches in philosophy as well divides between analytical, continental, and American traditions. They offer new interpretations of Aristotle, Confucius, Aquinas, Buridan, Kant, Pierce, Husserl, and Wittgenstein, and they challenge received views on normativity, the value of set theory, the objectivity of category schemes, and other topics. This volume, the first to offer a comprehensive examination of the subject, challenges mainstream positions on category theory. It will be of particular interest to philosophers and others concerned with how the world is divided.