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In philosophy, supervenience refers to a relation between sets of properties or sets of facts. X is said to supervene on Y if and only if some difference in Y is necessary for any difference in X to be possible.
Examples of supervenience, in which case the truth values of some propositions cannot vary unless the truth values of some other propositions vary, include:
Supervenience is of interest to philosophers because it differs from other nearby relations, for example entailment. Some philosophers believe it possible for some A to supervene on some B without being entailed by B. In such cases it may seem puzzling why A should supervene on B and equivalently why changes in A should require changes in B. Two important applications of supervenience involve cases like this. One of these is the supervenience of mental properties (like the sensation of pain) on physical properties (like the firing of 'pain neurons'). A second is the supervenience of normative facts (facts about how things ought to be) on natural facts (facts about how things are).
The possibility of "supervenience without entailment" or "supervenience without Reductionism" is contested territory among philosophers.
Its systematic use in philosophy is considered to have begun in early 20th-century meta-ethics and emergentism. As G.E. Moore wrote in 1922,"The Conception of Intrinsic Value", in Philosophical Studies, New York: Harcourt, Brace, and Co., pp. 253-75. "if a given thing possesses any kind of intrinsic value in a certain degree, then... anything exactly like it, must, under all circumstances, possess it in exactly the same degree" (p. 261). This usage also carried over into the work of R. M. Hare.For discussion of the emergentist roots of supervenience see Stanford Encyclopedia of Philosophy: Supervenience.
In the 1970s, Donald Davidson was the first to use the term to describe a broadly physicalist (and non-reductive) approach to the philosophy of mind, called anomalous monism. As he said in 1970, "supervenience might be taken to mean that there cannot be two events alike in all physical respects but differing in some mental respects, or that an object cannot alter in some mental respects without altering in some physical respects."Davidson, Donald (1970) "Mental Events." Reprinted in Essays on Actions and Events. Oxford: Clarendon Press
In subsequent years Terence ("Terry") Horgan, David Lewis, and especially Jaegwon Kim formalized the concept and began applying it to many issues in the philosophy of mind. This raised numerous questions about how various formulations relate to one another, how adequate the formulation is to various philosophical tasks (in particular, the task of formulating physicalism), and whether it avoids or entails reductionism.
(1) A-properties supervene on B-properties if and only if all things that are B-indiscernible are A-indiscernible. Formally:
For example, if one lets A be a set of mental properties, lets B be a set of physical properties, and chooses a domain of discourse consisting of persons, then (1) says that any two persons who are physically indiscernible are mentally indiscernible, and (2) says that any person who has a mental property has some physical property such that any person with that physical property has that mental property.
Some points of clarification: first, the definitions above involve quantification over properties and hence higher-order logic. Second, in (1), expressions of the form capture the concept of sharing all properties, or being indiscernible with respect to a set of properties. Thus, (1) can be understood more intuitively as the claim that all objects that are indiscernible with respect to a base set of properties are indiscernible with respect to a supervenient set of properties, or, as it is also sometimes said, that B-twins are A-twins. Finally, supervenience claims typically involve some modal force, however, the way that Modal logic force is specified depends on which more specific variety of supervenience one decides upon (see below).
(1) and (2) are sometimes called "schemata" because they do not correspond to actual supervenience relations until the sets of properties A and B, the domain of entities to which those properties apply, and a modal force have been specified. For modal forms of supervenience, the modal strength of the relation is usually taken to be a parameter (that is, the possible worlds appealed to may be physically possible, logically possible, etc.). Also, note that in the early literature properties were not always central, and there remain some who prefer to frame the relation in terms of predicates, , or entities instead, for example.
We can begin by distinguishing between local and global supervenience:
For example, if mental states locally supervene on brain states, then being in the same brain state entails being in the same mental state.
For example, if psychological properties globally supervene on physical properties, then any two worlds physically the same will be psychologically the same. The value of global supervenience is that it allows for supervenient properties to be determined not by local properties of an individual thing alone, but by some wider spatiotemporal distribution of things and properties. For example, something's being a dollar bill depends not only on the paper and the inks it is made out of, but also on a widely dispersed variety of features of the world it occupies.
Both local and global supervenience come in many forms. Local supervenience comes in strong and weak varieties:
The difference is essentially whether correlations between base and supervenient properties hold within actual worlds only, or across possible worlds. For example, if psychological properties strongly locally supervene on physical properties, then any two people physically the same, in any two worlds, will also be psychologically the same. On the other hand, if psychological properties only weakly locally supervene on physical properties, then those correlations between base and supervenient properties that hold in virtue of the supervenience relation are maintained within each world, but can be different in different worlds. For example, my physical duplicates in the actual world will have the same thoughts as I have; but my physical duplicates in other possible worlds may have different thoughts than I have in the actual world.
There are also several kinds of global supervenience relations, which were introduced to handle cases in which worlds are the same at the base level and also at the supervenient level, but where the ways the properties are connected and distributed in the worlds differ. For example, it is consistent with global mental–physical supervenience on the simple formulation described above for two worlds to have the same number of people in the same physical states, but for the mental states to be distributed over those people in different ways (e.g. I have my father's thoughts in the other world, and he has my thoughts). To handle this, property-preserving isomorphisms (1-1 and onto functions between the objects of two worlds, whereby an object in one world has a property if and only if the object which that function takes you to in the other world does) are used, and once this is done, several varieties of global supervenience can be defined.
Other varieties of supervenience include multiple-domains supervenience and similarity-based supervenience.
An alternative claim, advanced especially by John Haugeland, is a kind of weak local supervenience claim; or, weaker still, mere global supervenience. The claim that mental properties supervene globally on physical properties requires only a quite modest commitment: any difference between two possible worlds with respect to their instantiated mental properties entails at least some difference in the physical properties instantiated in those two worlds. Importantly, it does not require that the mental properties of an individual person supervene only on that person's physical state.
This weak global thesis is particularly important in the light of direct reference theories, and semantic externalism with regard to the content both of words and (more relevant to our concerns here) of thoughts. Imagine two persons who are indistinguishable in their local physical properties. One has a dog in front of his eyes and the other has a dog-image artificially projected onto his retinae. It might be reasonable to say that the former is in the mental state of seeing a dog (and of knowing that he does so), whereas the latter is not in such a state of seeing a dog (but falsely believes that he sees one).
There is also discussion among philosophers about mental supervenience and our experience of duration. If all mental properties supervene only upon some physical properties at durationless moments, then it may be difficult to explain our experience of duration. The philosophical belief that mental and physical events exist as a series of durationless moments that lie between the physical past and the physical future is known as presentism.
We can find supervenience 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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