Supervenience and Mind

This article is an informal introduction to the concept of supervenience and the role it plays in the philosophy of mind.  It surveys some of the many ways the concept has been used to reveal the manner and degree to which mental phenomena depend on facts about our bodies and their physical features. Philosophers usually construe the supervenience relation as a relation between classes of properties, where a class of properties, F, supervenes on a class of properties, G, just in case there is no difference in F-properties without some difference in G-properties.  As David Lewis puts it, “no difference of one sort without differences of another sort” (1986, p. 14).  It is in the philosophy of mind that we find the term’s most frequent contemporary occurrence.

The goal of asking whether one set of properties supervenes on another is to better understand the ontological relation between the two sets — especially, whether the one set of properties depends entirely on the other.  Suppose, for example, that two individuals can have different moral properties while being exactly alike in terms of their actual and potential behavior; that is, suppose that one’s moral features do not supervene on one’s behavioral features.  Then we can conclude that the former depend on something more than the latter.  And if we accept this conclusion, we are then led to search for a set of properties on which our moral features do supervene, a set of properties in terms of which any two individuals must differ with any moral difference.  The goal is to isolate just that set of features on which our moral properties do wholly rely.

Suppose we succeed in identifying a set of features on which F-properties supervene (where F-properties might be moral, mental, aesthetic, economic, or any other higher-level properties).  Then we can try to discover the nature of the dependence of F-properties on the underlying G-properties (for example, behavioral, physical, neurological, or intrinsic) by asking about the manner in which the former supervene on the latter.  Is it a logical truth that a difference in F-properties requires a difference in G-properties?  Is this covariance due to the causal laws that actually obtain?  Is it a matter of metaphysical necessity?  Asking these questions about the way in which F-properties supervene may help us decide whether the dependence is, for example, a wholly analytic affair, a type of causal dependence, a matter of constitution, or a matter of genuine identity.

Supervenience: The Basic Idea

Nora’s latest sculpture has many intrinsic features, including its shape, density, texture, and constituent matter.  It also has various aesthetic properties — beauty, grace, elegance, and expressive power.  No doubt, the aesthetic properties of the sculpture are in some way and to some degree a result of its intrinsic features.  But in what way exactly, and to what degree?  Thinking in terms of supervenience is a good start to finding the answer.  Imagine an artwork, x*, that is intrinsically indistinguishable from Nora’s sculpture, x — a perfect duplicate of x.  Is it possible that despite the indiscernability, x* might differ aesthetically from x?  If it is not possible for x and the intrinsically indiscernable x* to differ aesthetically, then we say that the object’s aesthetic features supervene on its intrinsic features, where a class of properties, F, supervenes on a class of properties, G, just in case a difference in F-properties requires a difference in G-properties; in other words, all the same G-properties guarantee all the same F-properties.

If we decide that the object’s aesthetic properties do supervene on its intrinsic features, then we are led to inquire whether the former are identical with the latter or whether the dependence relation is of some weaker sort — for example, causal dependence or constitution.  On the other hand, if we conclude that the object’s aesthetic properties do not supervene on its intrinsic features, that is, if x* might differ aesthetically from x despite their intrinsic similarity, then we can conclude that those aesthetic properties are at least partly a function of certain relations the object bears to external items.  We are then led to ask what the relevant external relations are.  We think in terms of supervenience again, imagining various changes in x’s environment (different origins, differences in historical context, different standards of the qualified judges, differences in popular opinion), and for each of those changes we decide whether x’s aesthetic features might also differ, until we isolate just those features of the environment on which the aesthetic properties do rely.  The conclusion would then be that the object’s aesthetic features supervene on its intrinsic properties together with those external features.

As this line of inquiry shows, the concept of supervenience is an invaluable tool for deciding whether and how one set of properties depends on another.  An analogous line of inquiry is found in discussions of mental content.  The content of one’s mental states depends largely on what the individual is like internally — on the state of the brain and the brain’s causal relations to other parts of the body, including sense organs and limbs.  But does the content of one’s mental states depend entirely on these intrinsic features?  To decide this issue, we consider whether it is possible for an indistinguishable individual, a molecule-for-molecule duplicate, to differ in terms of the content of her mental states.  And thanks to the thought experiments of Hilary Putnam (1973, 1975) and Tyler Burge (1979), it is widely thought that intrinsic duplicates can indeed differ in the content of their mental states.  Putnam has us imagine a twin-earth that is exactly like earth except that what they call “water” on twin-earth is comprised of something other than H2O molecules.  The content of your water-thoughts, it seems, differs from the content of your doppelganger’s “water”-thoughts on twin-earth — simply because of the difference in the liquid toward which those thoughts are directed.  (But note that Putnam’s example is actually designed to show a difference in linguistic content, meaning, which does not in itself entail a difference in mental content.)  Likewise, Burge shows that given suitable differences in surrounding linguistic practice, the thoughts one expresses with the word ‘arthritis’ might differ in content from those that one’s doppelganger expresses.  For example, if your duplicate inhabits a possible world in which ‘arthritis’ is regularly used to describe various conditions in addition to inflammation of the joints, then it seems that the content of the duplicate’s ‘arthritis’ thoughts will differ from yours.  Examples such as these seem to show that the content of one’s mental states does not supervene on one’s intrinsic features alone, but only on a set of features that includes features of one’s environment.

While Putnam and Burge do not use the term ‘supervenience’ in the essays mentioned above (though Burge does use it in his 1986 discussion of externalism), it is clear that the concept of “no difference of one sort without differences of another sort” is being utilized.  It was Davidson’s use of ‘supervenience’ in “Mental Events” (1970) that made the term popular in the philosophy of mind.