Term variously used.
For Rudolf Carnap (1891-1970), a member of the Vienna circle, it said that all scientific statements could be reduced to statements about ordinary physical objects (or else spatiotemporal points), such sentences having to be publically verifiable. For others it has meant that any meaningful statement can be translated into the language of physics. Currently, physicalism is most often used for the identity theory of mind, sometimes including behaviorism.
All these theories are forms of reductionism.
R Carnap, The Unity of Science (1932); translated with Introduction by M Black (1934)
Definition of physicalism
The word “physicalism” was introduced into philosophy in the 1930s by Otto Neurath and Rudolf Carnap.
The use of “physical” in physicalism is a philosophical concept and can be distinguished from alternative definitions found in the literature (e.g. Karl Popper defined a physical proposition to be one which can at least in theory be denied by observation). A “physical property”, in this context, may be a metaphysical or logical combination of properties which are physical in the ordinary sense. It is common to express the notion of “metaphysical or logical combination of properties” using the notion of supervenience: A property A is said to supervene on a property B if any change in A necessarily implies a change in B. Since any change in a combination of properties must consist of a change in at least one component property, we see that the combination does indeed supervene on the individual properties. The point of this extension is that physicalists usually suppose the existence of various abstract concepts which are non-physical in the ordinary sense of the word; so physicalism cannot be defined in a way that denies the existence of these abstractions. Also, physicalism defined in terms of supervenience does not entail that all properties in the actual world are type identical to physical properties. It is, therefore, compatible with multiple realizability.
From the notion of supervenience, we see that, assuming that mental, social, and biological properties supervene on physical properties, it follows that two hypothetical worlds cannot be identical in their physical properties but differ in their mental, social or biological properties.
Two common approaches to defining “physicalism” are the theory-based and object-based approaches. The theory-based conception of physicalism proposes that “a property is physical if and only if it either is the sort of property that physical theory tells us about or else is a property which metaphysically (or logically) supervenes on the sort of property that physical theory tells us about”. Likewise, the object-based conception claims that “a property is physical if and only if: it either is the sort of property required by a complete account of the intrinsic nature of paradigmatic physical objects and their constituents or else is a property which metaphysically (or logically) supervenes on the sort of property required by a complete account of the intrinsic nature of paradigmatic physical objects and their constituents”.
Physicalists have traditionally opted for a “theory-based” characterization of the physical either in terms of current physics, or a future (ideal) physics. These two theory-based conceptions of the physical represent both horns of Hempel’s dilemma (named after the late philosopher of science and logical empiricist Carl Gustav Hempel): an argument against theory-based understandings of the physical. Very roughly, Hempel’s dilemma is that if we define the physical by reference to current physics, then physicalism is very likely to be false, as it is very likely (by pessimistic meta-induction) that much of current physics is false. But if we instead define the physical in terms of a future (ideal) or completed physics, then physicalism is hopelessly vague or indeterminate.
While the force of Hempel’s dilemma against theory-based conceptions of the physical remains contested, alternative “non-theory-based” conceptions of the physical have also been proposed. Frank Jackson (1998) for example, has argued in favour of the aforementioned “object-based” conception of the physical. An objection to this proposal, which Jackson himself noted in 1998, is that if it turns out that panpsychism or panprotopsychism is true, then such a non-materialist understanding of the physical gives the counterintuitive result that physicalism is, nevertheless, also true since such properties will figure in a complete account of paradigmatic examples of the physical.
David Papineau and Barbara Montero have advanced and subsequently defended a “via negativa” characterization of the physical. The gist of the via negativa strategy is to understand the physical in terms of what it is not: the mental. In other words, the via negativa strategy understands the physical as “the non-mental”. An objection to the via negativa conception of the physical is that (like the object-based conception) it doesn’t have the resources to distinguish neutral monism (or panprotopsychism) from physicalism.
Supervenience-based definitions of physicalism
Adopting a supervenience-based account of the physical, the definition of physicalism as “all properties are physical” can be unraveled to:
1) Physicalism is true at a possible world w if and only if any world that is a physical duplicate of w is also a duplicate of w simpliciter.
Applied to the actual world (our world), statement 1 above is the claim that physicalism is true at the actual world if and only if at every possible world in which the physical properties and laws of the actual world are instantiated, the non-physical (in the ordinary sense of the word) properties of the actual world are instantiated as well. To borrow a metaphor from Saul Kripke (1972), the truth of physicalism at the actual world entails that once God has instantiated or “fixed” the physical properties and laws of our world, then God’s work is done; the rest comes “automatically”.
Unfortunately, statement 1 fails to capture even a necessary condition for physicalism to be true at a world w. To see this, imagine a world in which there are only physical properties—if physicalism is true at any world it is true at this one. But one can conceive physical duplicates of such a world that are not also duplicates simpliciter of it: worlds that have the same physical properties as our imagined one, but with some additional property or properties. A world might contain “epiphenomenal ectoplasm”, some additional pure experience that does not interact with the physical components of the world and is not necessitated by them (does not supervene on them). To handle the epiphenomenal ectoplasm problem, statement 1 can be modified to include a “that’s-all” or “totality” clause or be restricted to “positive” properties. Adopting the former suggestion here, we can reformulate statement 1 as follows:
2) Physicalism is true at a possible world w if and only if any world that is a minimal physical duplicate of w is a duplicate of w simpliciter.
Applied in the same way, statement 2 is the claim that physicalism is true at a possible world w if and only if any world that is a physical duplicate of w (without any further changes), is duplicate of w without qualification. This allows a world in which there are only physical properties to be counted as one at which physicalism is true, since worlds in which there is some extra stuff are not “minimal” physical duplicates of such a world, nor are they minimal physical duplicates of worlds that contain some non-physical properties that are metaphysically necessitated by the physical.
But while statement 2 overcomes the problem of worlds at which there is some extra stuff (sometimes referred to as the “epiphenomenal ectoplasm problem”) it faces a different challenge: the so-called “blockers problem”. Imagine a world where the relation between the physical and non-physical properties at this world (call the world w1) is slightly weaker than metaphysical necessitation, such that a certain kind of non-physical intervener—”a blocker”—could, were it to exist at w1, prevent the non-physical properties in w1 from being instantiated by the instantiation of the physical properties at w1. Since statement 2 rules out worlds which are physical duplicates of w1 that also contain non-physical interveners by virtue of the minimality, or that’s-all clause, statement 2 gives the (allegedly) incorrect result that physicalism is true at w1. One response to this problem is to abandon statement 2 in favour of the alternative possibility mentioned earlier in which supervenience-based formulations of physicalism are restricted to what David Chalmers (1996) calls “positive properties”. A positive property is one that “…if instantiated in a world W, is also instantiated by the corresponding individual in all worlds that contain W as a proper part.” Following this suggestion, we can then formulate physicalism as follows:
3) Physicalism is true at a possible world w if and only if any world that is a physical duplicate of w is a positive duplicate of w.
On the face of it, statement 3 seems able to handle both the epiphenomenal ectoplasm problem and the blockers problem. With regard to the former, statement 3 gives the correct result that a purely physical world is one at which physicalism is true, since worlds in which there is some extra stuff are positive duplicates of a purely physical world. With regard to the latter, statement 3 appears to have the consequence that worlds in which there are blockers are worlds where positive non-physical properties of w1 will be absent, hence w1 will not be counted as a world at which physicalism is true. Daniel Stoljar (2010) objects to this response to the blockers problem on the basis that since the non-physical properties of w1 aren’t instantiated at a world in which there is a blocker, they are not positive properties in Chalmers’ (1996) sense, and so statement 3 will count w1 as a world at which physicalism is true after all.
A further problem for supervenience-based formulations of physicalism is the so-called “necessary beings problem”. A necessary being in this context is a non-physical being that exists in all possible worlds (for example what theists refer to as God). A necessary being is compatible with all the definitions provided, because it is supervenient on everything; yet it is usually taken to contradict the notion that everything is physical. So any supervenience-based formulation of physicalism will at best state a necessary but not sufficient condition for the truth of physicalism.
Additional objections have been raised to the above definitions provided for supervenience physicalism: one could imagine an alternate world that differs only by the presence of a single ammonium molecule (or physical property), and yet based on statement 1, such a world might be completely different in terms of its distribution of mental properties. Furthermore, there are differences expressed concerning the modal status of physicalism; whether it is a necessary truth, or is only true in a world which conforms to certain conditions (i.e. those of physicalism)