supervenience - A set of properties or facts M supervenes on a set of properties or facts P if and only if there can be no changes or differences in M without there being changes or differences in P.
The notion of supervenience was first introduced into the philosophy of mind by Donald Davidson (1970):
"[M]ental characteristics are in some sense dependent, or supervenient, on physical characteristics. Such supervenience might be taken to mean that there cannot be two events alike in all physical respects but differing in some mental respect, or that an object cannot alter in some mental respect without altering in some physical respect. (p. 98)"
An example of supervenience is given in the relations between the acceleration, velocity, and position of an object in space. An object cannot change its acceleration without changing its velocity, and in turn, cannot change its velocity without changing its position. Thus, facts about an object's acceleration supervene on facts about an object's velocity which in turn supervene on facts about an object's position. The example of moving objects illustrates an important feature of supervenience, namely, that supervenience is transitive. Another important feature of supervenience illustrated by this example is that the supervening properties need not be identical to the properties upon which they supervene. Thus, acceleration supervenes on velocity, but is not the same thing as velocity.
The fact that supervening properties need not be identical to their subvening properties is the source of the great appeal of supervenience to contemporary philosophers of mind who have come to think that the mental cannot be identical to the physical (largely due to considerations of multiple realizability) yet want to be physicalists and thus hold on to the notion that the mental is nonetheless determined by the physical. Thus they subscribe to the thesis of psychophysical supervenience, AKA, the supervenience thesis.
The supervenience thesis states that mental properties and facts supervene on physical properties and facts. The supervenience thesis can be further unpacked as the following three theses about objects and their properties. (Please note that I use the term "object" as a place holder for the unwieldy "object, event, or state of affairs" and the term "properties" to denote extrinsic as well as intrinsic properties.)
(i) No two objects can differ in their mental properties without differing in their physical properties.
(ii) A single object cannot change its mental properties without changing its physical properties.
(iii) If, at a given time t, a single object has two different subsets of mental properties, it must have two different subsets of physical properties.
The above three corollaries to the supervenience thesis are each captured by the slogan "no mental differences without physical differences."
Variations of the supervenience can be obtained by (1) varying the modal force of the thesis and (2) by varying the "objects" compared.
Regarding (1), versions of the thesis that are to apply to objects in only a single possible world are known as 'Weak' Supervenience, whereas versions that allow for comparisons across possible worlds are known as 'Strong' Supervenience.
Regarding (2), various versions of the thesis quantify over, from smallest to largest, persons, spatial regions, or whole possible worlds, yielding 'Local', 'Regional', and 'Global' Supervenience, respectively.
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