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In philosophy, supervenience is a well-defined relation between "higher-level" (e.g. mental) and "lower-level" (e.g. physical) properties. Informally, a group of properties X supervenes on (alternatively, is supervenient on) a group of properties Y exactly when the X-group properties are determined by the Y-group properties, where "determined by" is taken somewhat non-specifically (weakly?).
Formally, X-group properties supervene on Y-group properties if and only if any of the following holds for all objects a and b:
- a and b cannot differ in their X-group properties without also differing in their Y-group properties.
- If a and b have identical Y-group properties, then they also have identical X-group properties.
- If a and b do not have identical X-group properties, then they also do not have identical Y-group properties.
(All of these formulations are logically equivalent, so if one of them holds, all of them do.)
Examples
Value The value of a physical object to an organism is sometimes held to be supervenient upon the physical properties of the object.
Philosophy of mind In philosophy of mind, several people argue that mental properties supervene on physical properties. In its most recent form this position derives from the work of Donald Davidson, though it has largely been advanced by others. This can be taken in several senses. The simplest is perhaps the claim that the mental properties of a person supervene on that person's physical properties. Then: - An atom-by-atom replication of a person will have the same mental characteristics as the original. (This follows from the second formulation above.)
- Two different people (i.e., two people who have two different minds) must have two physically different brains. (This follows from the third formulation above.)
An alternative, advanced especially by John Haugeland, is called "weak supervenience," or, at its weakest, "global supervenience." To claim that mental properties globally supervene on physical properties is to claim only that a change in the mental properties instantiated between two different possible worlds requires some change in the physical properties instantiated in those two worlds. Specifically, it does not claim that the mental properties of an individual person supervene only on that person's physical state.
The latter, weaker thesis, is particularly important in the light of direct reference theories and semantic externalism with regard to the content both of words and of thoughts. For example, imagine two physically identical people, one of them looking at a dog and the other having a dog-image projected onto his retinae. It might be reasonable to say that the former is in the mental state of "perceiving a dog," whereas the latter is not and merely (falsely) believes that he is.
There is also discussion amongst philosophers about supervenience and time. If mental properties only supervene on a durationless instant then it is difficult to explain the experience of duration. The philosophical belief that mental and physical events only exist for the zero seconds that lie between the physical past and the physical future is known as presentism and is a form of belief in Galilean relativity.
There are several examples of supervenience to be found in computer networking. For example, note that in a dial-up internet connection, an audio signal in a phone line transports IP packets between the user's computer and the Internet service provider's computer. In this case, the arrangement of bytes in that packet supervenes on the physics of the phone signal. More generally, each layer of the OSI Model of computer networking supervenes on the layers below it.
These computer examples exemplify a more general principle: we will find supervenence wherever a message is conveyed by a representational medium. When we see a letter 'a' in a page of print, for example, the meaning "latin lowercase a" supervenes on the geometry of the boundary of the printed glyph, which in turn supervenes on the ink deposition on the paper.
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