This is familiar to most of you, but I am including it in a paper I am writing.
The Concept of Naturalism
Exactly what does Lewis mean by naturalism? Very often the terms Naturalism and Materialism are used interchangeably, but at other times it is insisted that the two terms have different meanings. Lewis says,
“What the naturalist believes is that the ultimate Fact, the thing you can’t go behind, is a vast process of time and space which is going on of its own accord. Inside that total system every event (such as your sitting reading this book) happens because some other event has happened; in the long ru, because the Total Event is happening. Each particular thing (such as this page) is what it is because other things are what they are; and so, eventually, because the whole system is what it is.”22
As a presentation of naturalism, however, this might be regarded as inadequate by contemporary naturalists, because it saddles the naturalist with a deterministic position. The mainstream position in contemporary physics involves an indeterminism at the quantum-mechanical level. Lewis himself thought that this kind of indeterminism was really a break with naturalism, admitting the existence of a lawless Subnature as opposed to Nature, but most naturalists today are prepared to accept quantum-mechanical indeterminism as part of physics and do not see it as a threat to naturalism as they understand it. Some critics of Lewis have suggested that his somewhat deficient understanding of naturalism undermines his argument. Lewis, however, insisted on “making no argument” out of quantum mechanics and expressed a healthy skepticism about making too much of particular developments in science that might be helpful to the cause of apologetics.23
However, contemporary defenders of the Argument from Reason such as William Hasker and myself have developed accounts of materialism and naturalism that are neutral as to whether or not physics is deterministic or not. Whatever Lewis might have said about quantum-mechanical indeterminacy, the problems he poses for naturalism arise whether determinism at the quantum-mechanical level is true or not.25
Materialism, as we understand it, is committed to three fundamental theses.
1) The basic elements of the material or physical universe function blindly, without purpose. Man is the product, says Bertrand Russell, of forces that had no prevision of the end they were achieving. Richard Dawkins’ exposition and defense of the naturalistic world view is called The Blind Watchmaker: Why the Evidence of Evolution Reveals a World Without Design26 not because no one ever designs anything in a naturalistic world, but because, explanations in terms of design must be reduced out in the final analysis. Explanation always proceeds bottom-up, not top-down.
2) The physical order is causally closed. There is nothing transcendent to the physical universe that exercises any causal influence on it.
3) Whatever does not occur on the physical level supervenes on the physical. Given the state of the physical, there is only one way the other levels can be.
The argument from reason is concerned with what philosophers today call prepositional attitudes, states such as believing that proposition p is true, desiring that proposition p be true, doubting that proposition p be true, etc, and how these prepositional attitudes come to be caused. There are three types of materialism, at least so far as propositional states are concerned. One type is called eliminative materialism, which argues that propositional attitudes like belief and desire do not exist. Another kind is called reductive materialism, according to which mental states can be analyzed or reduced in physical terms. A third kind is non-reductive materialism, according to which mental states are not to be analyzed in physical terms, but given the state of the physical, there is only one way the mental can be. All of these positions are consistent with the definition of materialism given above.
As for naturalism, it is hard to see how a world-view could be naturalistic without satisfying the above definition. Perhaps a world could be naturalistic if there was no matter or if the science describing the most basic level of analysis is not physics. However, whatever objections there might be to materialism based on the argument from reason would also be objections to these forms of naturalism. So although the argument is primarily directed at materialism, so far as I can tell, there is no form of naturalism that fares any better against the argument from reason than materialism.