In metaphysics, particulars or individuals are usually contrasted with universals.[1][2] Universals concern features that can be exemplified by various different particulars. Particulars are often seen as concrete, spatiotemporal entities as opposed to abstract entities, such as properties or numbers. There are, however, theories of abstract particulars or tropes. For example, Socrates is a particular (there's only one Socrates-the-teacher-of-Plato and one cannot make copies of him, e.g., by cloning him, without introducing new, distinct particulars). Redness, by contrast, is not a particular, because it is abstract and multiply instantiated (for example a bicycle, an apple, and a particular woman's hair can all be red). In the nominalist view, everything is particular. A universal at each moment in time, from the point of view of an observer, is a set of particulars.