A distinction is often drawn in philosophy between two types of objects of awareness in perception. First, there are physical objects or substances (such as chairs, books, rocks, and water) and living organisms (animals, plants, and human beings insofar as they are perceptible, that is, their bodies). A common technical term for all these is material objects. Second, there are data of immediate awareness, which we shall refer to as "sensa" (singular, sensum), such as color patches or shapes, sounds, smells, and tactile feelings. This distinction is usually fourfold: (a) in status—material objects are external, located in physical space, and "public" (observable by different persons at once), while sensa are private and are usually held to have no external physical existence; (b) in extent—material objects may at one time correspond to several sensa and normally persist throughout the occurrence of many sensa; (c) in directness—the...