  Apart from his Materialistic or Spatio-Temporal Naturalism, Williams’s major contributions to metaphysics lie in the realm of ontology, and concern the fundamental constituents of Being. His key proposal is that properties are indeed real—in fact Reality consists in nothing but properties—but that these properties are not Universals, as commonly supposed, but particulars with unique spatio-temporal locations. The structure of Reality comprises a single fundamental category, Abstract Particulars, or “tropes”. Tropes are particular cases of general characteristics. A general characteristic, or Universal, such as redness or roundness, can occur in any one of indefinitely many instances. Williams’ focus was on the particular case of red which occurs as the color, for example, of a particular rose at a specific location in space and time, or the particular case of circularity presented by some particular coin in my hand on a single, particular occasion. These tropes are as particular, and as grounded in place and time, as the more familiar objects, the rose and the coin, to which they belong.