In the philosophy of mind, functionalism is the modern successor to behaviourism. Its early advocates were Putnam and Sellars, and its guiding principle is that we can define mental states by a triplet of relations: what typically causes them, what effects they have on other mental states, and what effects they have on behaviour. The definition need not take the form of a simple analysis, but if we could write down the totality of axioms, or postulates, or platitudes that govern our theories about what things are apt to cause (for example) a belief state, what effects it would have on a variety of other mental states, and what effects it is likely to have on behaviour, then we would have done all that is needed to make the state a proper theoretical notion. It would be implicitly defined by these theses. Functionalism is often compared with descriptions of a computer, since according to it mental descriptions correspond to a description of a machine in terms of software, that remains silent about the underlying hardware or ‘realization’ of the program the machine is running. The principal advantages of functionalism include its fit with the way we know of mental states both of ourselves and others, which is via their effects on behaviour and other mental states. As with behaviourism, critics charge that structurally complex items that do not bear mental states might nevertheless imitate the functions that are cited. According to this criticism functionalism is too generous, and would count too many things as having minds. It is also queried whether functionalism is too parochial, able to see mental similarity only when there is causal similarity, when our actual practices of interpretation enable us to ascribe thoughts and desires to persons whose causal structure may be rather different from our own. It may then seem as though beliefs and desires can be ‘variably realized’ in causal architectures, just as much as they can be in different neurophysiological states. See also homuncular functionalism, physicalism, Turing test.