Nominalism is better characterised not as a single metaphysical doctrine, but as a family of heterogeneous theories which share a reluctance to accept the existence of universals and/or of abstract entities. Since one may accept abstract particulars or concrete universals, the rejection of universals and the rejection of abstract entities amount to different positions. Nominalism often has been associated with many other philosophical positions, such as empiricism, materialism, extensionalism, reductionism, and particularism. Strictly speaking, such connections are not in all cases necessary, but they are very common in the history of philosophy. Mereology is interesting for nominalists for at least two reasons. The first one is that mereology provides an alternative primitive relation to the one traditionally offered by realists. Realists usually see something like participation or instantiation as the primitive ontological relation: take the true sentence ‘a is F’. It expresses the basic fact that a particular thing a instantiates the property F. Given that participation in one universal is supposed to explain such phenomena as natural classes and universal predication, the predicate ‘F’ is naturally taken to express a universal property. Thus, realism is committed to two …

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