ABSTRACT
Biologists, chemists, physicists, zoologists, and many others supply people with some provisional answers: There are trees, cells, bosons, horses, and so on. Reasoning about the commonsense beliefs can supply people with further answers. Ontologists look more deeply into such provisional answers and ask which of them are true and which only appear to be true. Abstract objects are things like the number two, the set of all horses, beauty, and so on. They are the sorts of things that cannot even in principle be detected by sensory experience or instruments. If they exist at all, they are immaterial, have no location in spacetime, and lack causal powers. Sentences are not normally thought of as having referents, but many philosophers do believe that typical declarative sentences, and maybe others as well, express things in the way that predicates or names do. Unicorns look like horses, but have a horn.
