Ontology is the philosophical discipline that tries to find out what there is: what entities make up reality, what is the stuff the world is made from? Thus, ontology is part of metaphysics, and in fact it seems to be about half of all of metaphysics. It tries to establish what (kinds of) things there are, the other half tries to find out what the (general) properties of these things are and what (general) relations they have to each other. Settling questions in ontology would bring with it major progress in metaphysics. And it would bring with it major progress in a variety of areas in philosophy outside of metaphysics as well. Many philosophical debates outside of metaphysics are quite directly influenced by how things turn out in ontol-ogy. Whether or not there are certain entities will give rise to quite different answers in various philosophical debates. I would like to very briefly describe three debates where ontological questions play a central role. These debates are either directly from metaphysics or from other areas of philosophy and they will be of central importance throughout this paper: 1. Numbers One of the main questions in the philosophy of mathematics is the question whether or not there is a domain of mathematical objects that mathematics aims to correctly describe. Depending on what one says about this, it will give rise to quite different conceptions of the nature of mathematics, the nature of mathematical objectivity, the nature of mathematical knowledge, and so on. Thus, if there are (natural) numbers, if reality contains such entities, then mathematics, or at least arithmetic, will be about a domain of entities, and this will bring with it certain views about the nature of arithmetic. To settle the question about the existence of numbers would thus be major pro-gress in the philosophy of mathematics.
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