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Supervenience

Supervenience


Ever since Donald Davidson introduced the notion of supervenience within the philosophy of mind in 1970, it has come to play a key role in philosophical discussions regarding reducibility and the ontological structure of the world in general. With its help, philosophers of mind, in particular, hoped to solve the question of whether a nonreductive kind of materialism could be upheld that would avoid the pitfalls of traditional dualism on the one hand and of traditional materialism on the other. The key attraction of supervenience was that it seemed to deliver dependence without reduction. Events at higher levels of reality could thus be seen as totally determined by lower-level events without higher-level laws of the so-called special sciences being reducible to physics.


Definition and types

In a loose sense, the core idea is as follows: B-properties (e.g., mental properties) supervene on A-properties (e.g., physical properties) if any two possible situations identical in their A-properties are identical in their B-properties. In other words, the B-facts "come along with" the A-facts, hence supervenience. The term is derived from the Latin words venire (to come) and super (on top of). Thus, if B-properties supervene on A-properties, then once the A-facts are fixed, the B-facts are fixed. They are automatically put in place as soon as the lower-level properties are put in their place. This is because, where supervenience reigns, there is no room for variation of the higher-level properties independently of the lower-level (e.g., physical) properties. In this way supervenience yields ontological dependency relations. At the same time supervenience has also been supposed to bar reducibility, thus freeing the way for a novel nonreductive brand of materialism in the philosophy of mind and, more generally, allowing special sciences to be autonomous without abandoning, as in traditional dualism, a unified materialist picture of the world.

The dependency relations enabled by supervenience may in fact vary in strength depending on the specific kind of supervenience relation involved. The above definition of supervenience, in effect, generates in its turn four different kinds of supervenience relations. First, the word situation as used in that definition may refer either to individuals or to entire worlds. Accordingly, local and global supervenience must be distinguished. Secondly, the word possible may refer to either logical or nomological possibility, giving rise to logical versus natural supervenience.

For the former distinction between local and global supervenience, consider an animal and its molecular "twin" (a molecule-by-molecule replica of the given animal). Although they must share the same shape, they do not necessarily share the same degree of fitness, since they do not necessarily share type-identical environments. Hence, shape, but not fitness, supervenes locally on physical properties. However, fitness does supervene globally on physical properties. When all the physical facts of this world are duplicated so that molecular twins will also be located in type-identical habitats, then physical duplicates must share exactly the same degree of fitness.

Clearly, local supervenience is the stronger of the two supervenience relations. Properties that are locally supervenient must also be globally supervenient, but not vice versa. Conversely, many more properties turn out to be dependent on lower-level properties when considered under broad conditions of global supervenience than when they are considered in local isolation, so to speak, regardless of context. Thus it can easily be seen that if we duplicate all the physical facts of the entire universe down to the minutest details of the distribution of microphysical properties in space and time (and we do nothing else), then all the biological facts of our world will be duplicated as well. Since the physical "recipe" of our world fixes all its objects, including the way they move and function and the way they physically interact, it, in effect, also fixes the biological facts. Even God could not have created a world that was physically identical to ours but biologically distinct. There simply is no logical space for the biological facts to vary independently of the physical facts of our world when considered in toto. Furthermore, since this holds for any logically possible physical duplicate of our world, it follows that biological properties logically (globally) supervene on physical properties.

This is a remarkable result. One may well wonder whether under such broad conditions of global supervenience there can be any property at all that could fail to supervene on the (micro-) physical facts of an entire world. If not, physicalist materialism would carry the day. This brings us to the second distinction mentioned above, the distinction between logical and natural supervenience. This is the more interesting distinction because it leads straight into highly controversial territory. Generally speaking, B-properties naturally supervene on A-properties if any two naturally possible situations with the same A-properties also have the same B-properties. In other words, in the case of natural supervenience, the B-facts are nomologically, though not logically, implied by the A-facts. That is to say, in possible worlds that are governed by the same natural laws as the actual world, the A-facts naturally necessitate the B-facts (assuming natural supervenience). Clearly, natural possibility is much stricter than logical possibility. For example, a (stable) cubic kilometer of uranium-235 is logically, but not naturally, possible. The critical question then is: Are there any (higher-level) properties that accompany the physical facts in all naturally possible worlds without being fixed by the physical facts in all logically possible worlds? And the controversial answer given by some philosophers is: Consciousness is such a property. On the one hand, they argue, consciousness at least naturally supervenes on the physical facts because any two physically identical creatures in the natural world will presumably have qualitatively identical phenomenal experiences. Nevertheless, these philosophers go on to argue, consciousness fails to supervene logically on the physical facts of our world. Here they appeal to two famous thought experiments. It seems entirely conceivable that a creature physically identical to a conscious creature might lack consciousness altogether (like a zombie), or might have experiences qualitatively very different from ours (they might have so-called inverted qualia, so that, for example, they might have our sensation of phenomenal red when looking at the sky). Therefore, if these two intuitions hold, materialism is false. A full account of the physical facts of our world, including a specification of the minutest details of the distribution of its microphysical properties over space and time, would yet leave entirely undetermined the quality, even the existence, of the phenomenal properties of our world.

Principle of multiple realizability

As discussed above, supervenience may yield ontological dependency relations of varying degrees. But how do things stand with respect to the other philosophical benefit reputedly reaped from this recently developed notion of supervenience, that of barring reducibility? This becomes apparent when one focuses on the converse relation implied in the definition of supervenience. Assuming supervenience, while two situations cannot differ in their B-properties without a corresponding difference in their A-properties, the converse does not hold. That is to say, type-identical B-situations may be realized by an indefinite variety of type-different A-situations. In other words, the notion of supervenience brings in its wake an important corollary notion, that of multiple realizability. And again, this feature holds special interest for the philosophy of mind because multiple realizability is just what we expect in the mental realm. Pain in humans may be realized by C-fibers firing, while in dolphins it may be realized by D-fibers firing without ceasing to be just another simple instance of pain. Indeed, the situation may be vastly more diverse and confusing at the physical level than this example suggests: Your headache may be physically realized differently from mine, as, for that matter, may my headache today versus my headache tomorrow. Similarly, the property of being a monetary transaction, which is a unitary concept at the level of economics, may be physically realized by a wide variety of physical events lacking any perspicuity or explanatory integrity at the level of physics. Accordingly, the predicates of a given special science will only map onto predicates of physics that are at best wildly disjunctive. Thus, inasmuch as supervenience entails multiple realizability, higher-level supervening properties turn out to be irreducible. In psychology, in particular, it has been argued, there cannot be any type-type identities between mental properties and the physical properties realizing them. Nor, consequently, is there any room for strict psychophysical laws so as to reduce psychology to neurophysiology and ultimately to physics. In general, supervenient properties, in spite of being ontologically dependent upon their subvenient base properties, retain their ontological and explanatory autonomy.


Principle of multiple supervenience

The above argument for irreducibility appealed to supervenience in connection with the corollary notion of multiple realizability. More recently, however, supervenience has also been invoked in an antireductionist line of argument deemed to be more effective, in which the crucial corollary notion was not multiple realizability but rather multiple supervenience. An analogy with dispositional properties may clarify the point. Usually one and the same categorical base may "realize" more than one disposition. Even so, only one of those will be causally relevant for a given event. Thus, Sally's death is related to the electrical conductivity of her aluminum ladder. But the categorical base thereof (the cloud of free electrons permeating the metal) also "realizes" such diverse dispositions as the thermal conductivity or the opacity of the metal. Clearly, the correct explanation for the tragic accident would be the one that cited the relevant disposition, not just the categorical base property. Thus explanations couched in terms of supervenient properties (dispositional or otherwise) cannot be reduced to explanations citing no more than the corresponding base properties. But the important point in this context is that the irreducibility in question clearly does not consist in the fact that these higher-level properties may be multiply realizable, but precisely in the opposite fact that a given categorical base property does not identify which of the higher-level properties realized is explanatorily relevant in a given case. In other words, it is not multiple realizability but rather multiple supervenience of macroproperties onto one and the same subvenient categorical base that necessitates citing that supervenient property, which is responsible for the given effect. Similarly, if the aluminum ladder had been exposed to the heat of the sun for a while, it would have been the thermal conductivity, not the electrical conductivity, that would have been causally responsible for Sally's burning her feet, as the case might have been, even though either disposition is realized in the very same categorical base. Thus the special character of higher levels of organization in nature can be vindicated in principle, and perhaps even more effectively, by invoking multiple supervenience in addition to the more conventional appeal to multiple realizability. Such hierarchical levels necessitate the need for macroexplanations with the causal depth and the theoretical appropriateness corresponding to the grain of the explanatory level in question.

In sum, supervenience affords the insight that macrocausation is a real force in nature at multiple levels of existence. Consequently, downward causation may be assigned a stable place in our picture of how the world is organized without upsetting our conception of the various domains of physics as constituting a closed and complete system of physical events at the physical level of description.

See also Consciousness Studies; Downward Causation; Mind-body Theories; Mind-brain Interaction; Neurosciences


Bibliography

chalmers, david. the conscious mind: in search of a fundamental theory. new york: oxford university press, 1996.

davidson, donald. "mental events." in experience and theory, eds. l. foster and j. w. swanson. amherst, mass.: 1970.

fodor, jerry a. "special sciences." synthese 28 (1974): 77115.

kim, jaegwon. "supervenience." in a companion to the philosophy of mind, ed. samuel guttenplan. cambridge, mass.: blackwell, 1994.

meyering, theo c. "mind matters: physicalism and the autonomy of the person." in neuroscience and the person: scientific perspectives on divine action, eds. robert russell; nancy murphy; theo c. meyering; and michael arbib. berkeley: vatican observatory and center for theology and the natural sciences, 1999.

savellos, elias e., and yalcin, Ümit d., eds. supervenience: new essays. cambridge, uk: cambridge university press, 1995.

theo c. meyering

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