Descriptions of a thing at one level can be true in virtue of some underlying properties, in the way that the description of a computer as running such-and-such a program is true in virtue of the configurations of its circuitry. It may then be true that other configurations at the lower level could have had the same result, and where this is so it is said that the higher-level properties can be variably realized by the lower-level ones. In the philosophy of mind, it is plausible to think that a person's psychological states could be realized by a variety of different configurations at the neural level. The point is sometimes used to argue the superiority of functionalism over mind–brain identity theories.
Philosophy dictionary. Academic. 2011.