Naturalism, Materialism, Physicalism, Oh My!

This post is part of a series on naturalism. The first post is here.

I noted in that post Trent Dougherty's complaint that he couldn't find a decent defense of naturalism anywhere.

Has anyone given a decent academic case for naturalism? The closest I can recall is Melnyk's in his Material Manifesto. Maybe David Papineau's closure argument could be generalized. I think naturalists just assume it's all going to work out. It just seems utterly hopeless to me.

Despite Trent's complaints, I had little trouble finding serious philosophical defenses of naturalism. I am working my way through two of them: Physicalism, by Jeffrey Poland, and A Physicalist Manifesto, by Andrew Melnyk. Trent seems to have read the latter (though he got the title wrong), but gives no clue about what he found lacking in its presentation.

I am going to try to summarize the approaches taken by Poland and Melnyk. But first, why "physicalism" rather than "naturalism"?

According to the Stanford Encyclopedia of Philosophy, naturalism "has no very precise meaning in contemporary philosophy." It suggest a commitment to natural, as opposed to supernatural, entities and explanations. But, the SEP notes, "the great majority of contemporary philosophers would happily accept naturalism as just characterized," without necessarily accepting such ideas as being able to reduce mental phenomena to physical phenomena.

Materialism and physicalism are sometimes used synonymously. Materialism is sometimes taken to imply that only matter truly exists. Since Einstein discovered that E = mc^2 over 100 years ago, it is hard to believe that anyone would make this claim today. In contemporary understanding, matter and energy are interchangeable; they are, in fact, two aspects of the same thing. Thus, the materialist should at least expand the physical basis to include energy (or perhaps the energy-momentum tensor). Too, the term "materialism" has a variety of other uses, including Marxist and eliminative, which is perhaps why those writing about materialism in the current sense prefer the term "physicalism" (even though my spell checker doesn't think it's a word).

Physicalism, very roughly, is the idea that everything that exists is in some sense dependent on a physical substrate. But how to make this more precise? In what sense should higher-order phenomena (anything from a chair to the concept of justice in the mind of Antonin Scalia) depend on the purely physical facts about the world? Philosophers have proposed many different types of dependence.

• Identity: Everything that exists can be identified with some subset of the physical world.
• Definition: Everything that exists can be defined in the language of physics.
• Derivation: The laws of the higher-order sciences can be derived from the laws of physics.
• Supervenience: Any possible world that is physically indiscernible from the actual world is, in fact, completely indiscernible from the actual world.

While each of these captures some aspect of the physicalist thesis, Poland and Melnyk agree that none of them is completely satisfactory. Some are too strong: it seems highly unlikely that any purely physical system can be identical to the concept of justice in the mind of Antonin Scalia. Others are too weak: the indiscernibility requirement of supervenience can't guarantee that non-physical facts can be explained in terms of physical facts.

A better formulation, according to both authors, is given by realization physicalism: everything that exists is either physical, or is realized by a physical property or system.

That means we need to answer two questions. What does it mean for something to be physical? And what does it mean to be realized by something physical?

Next time: Physics is Fundamental!