The study of ontology is broad, while other branches of metaphysics are more specific. One main question in ontology are what the most basic categories of being are. For example, imagine a tree. A tree is part of a larger category, like a plant. Plants are part of larger category, too: living things. Eventually, we arrive at a very large category: "substance". Philosophers who study ontology want to discover and understand basic categories like substance. Aristotle, for example, tried to understand reality through many other categories that he discovered, such as Quantity, Quality, Relation, Place, and Time among others.