This post can be considered an extended application of the theory I described yesterday, which proposed a way to evaluate and compare philosophical theories based on their content alone. Here then I will apply that method to dualism as a theory about the mind, both as an indirect argument in favor of that method of evaluation (because it rejects dualism, which we are already motivated to reject for other reasons), and as a way of further detailing how exactly theories should be evaluated by providing an example of just such an evaluation.*
Let’s start with one of the ways in which dualism is claimed to be superior to materialism. Dualism, it is often argued, is to be preferred over materialism because materialism can’t explain various mental phenomena, usually intentionality and qualia. Now under this evaluative scheme this is in fact a valid complaint. If a theory about the mind can’t answer “why does X feel like it does?” then it has left some questions unanswered. Now let’s set aside whether materialism really can’t answer such questions, because to get into that would involve trying to answer them, and that would take all day. Instead it suffices to point out that dualism can’t answer them either. Let’s say that a specific input feeling like red is explained by appeal to some non-physical mental stuff that makes it feel like red. But why then does that mental stuff make it feel like red (rather than, say, green)? That is a question that dualism is equally unable to answer. And in fact the answer it does give may fail the content criterion (it could equally well explain why that input feels like green or blue). So dualism leaves just as many questions unanswered as materialism does, and so on these topics it is no better than even the most simplistic materialism as an explanation.
Dualism, in most of its forms, also leaves unanswered the question “who has a mind?” which materialism does answer. Materialism automatically answers this question because it identifies the mind with some physical stuff or operation of the physical stuff. Thus who has a mind is a question that can be answered by studying the physical properties of things to determine if they have the right stuff or operate in the right way. Now a dualist theory could postulate some additional law governing which physical systems mental stuff accompanies. That does answer this particular question, but leaves open another question, which I will raise below.
To actually explain the mind, assuming that we have a dualist theory which goes beyond a vague appeal to mental stuff, the dualist theory must posit theoretical entities that interact so as to explain the mind, intentionality, experience, and so on. Let’s consider a very simple dualist theory that posits that the mental stuff is divided into two parts, which can be in a number of different states, and that which change state based on their interactions. These two parts and their interactions then supposedly explain the mind. We could say: because part A was in such and such state, and caused part B to be in some other state we experienced X. This is a pattern that must in fact be followed by every explanation in every domain; a number of entities (possibly things, possibly properties) are posited and then the state of those entities and the rules governing them act as an explanation. But here we run into problems for dualism. Nothing stops us from identifying those entities, their states, and their interactions with some of the physical components of the brain (well, in theory you could argue that the brain just doesn’t work that way, that no corresponding parts exist, but we don’t know enough about the details of the brain to possibly prove such a claim yet). Thus every successful dualist explanation could equally well be a materialist explanation. (I call this the Chalmers problem. Chalmers posits that the non-physical mental properties parallel the information processing properties of the system. But if they parallel them perfectly, and thus explain the mind, why not just identify them?) But the dualist explanation posits something more than the materialist version of the same theory does: it must posit additional laws governing a new domain of mental stuff that makes it behave in this way and stick to the right sort of physical systems. (The materialist version obviously can just invoke the laws of physics and the structure of the brain.) Now we might be tempted to ask “why do these laws obtain?”, but that is a nonsensical question. If they really are fundamental laws, like the laws of physics are, such a question doesn’t make sense. But we can ask “how could we know that these laws obtain?”. And the dualist cannot give an answer to this question because they can’t know that the laws obtain.
Now of course we wouldn’t expect dualism to answer the question in the same way that materialism would. Materialism would appeal to neuroscience, and say that we know that the rules obtain because of our knowledge of neuroscience (or will know in the future when neuroscience develops further). Essentially then materialism answers the question by appealing to another discipline. Dualism is thus more like physics with respect to this question, as already indicated. And physics answers it by an appeal to the best explanation, saying that the physical laws are the best description of the observations made so far. But the laws of the non-physical domain cannot be known in this way, not even by appealing to our own mental life as evidence. Because our own mental life could be explained equally well by the materialist counterpart of the dualist theory, the one that identifies the theoretical entities with some facts about physical stuff rather than non-physical mental stuff. So while we might be able to know that the mental laws are such-and-such we could never know that they are the laws of a non-physical domain.
Now I wouldn’t say that these problems are unique to dualism. Any time we posit non-physical stuff as an attempt at an explanation we are going to run into basically the same problems. For example, Frege posited the sense to explain reference. But that theory can’t answer the question “why does a particular sense refer to what it does?”, just as dualism can’t say why the non-physical stuff is responsible for qualia and intentionality. And if the sense is supposed to be non-physical then how do we know that senses behave as the theory posits that they do? Obviously any evidence we obtain based on our observations of language use could equally well supports a theory that identified the sense with some feature of the language center of the brain, and that theory could confirm that the senses actually behaved as we posit they do by studying the physical stuff they are identified with directly. So, as a broader claim, we might very well conclude that positing non-physical stuff in general isn’t good philosophy.
* Of course here I am picking on dualism again. Why do I pick on dualism so much? It’s not because the possibility that dualism might be true worries me. Although we don’t have a complete theory of mind yet the progress already made towards one strongly suggests that materialism is true, as neuroscience and psychology pull closer and closer together. Rather I tend to pick on dualism because the philosophy of mind is my area of expertise, and thus I judge that I am least likely to make a mistake while picking on it. And occasionally I do need to pick on a philosophical position in order to test whether a certain argumentative strategy works or not, just as a logician might test out a new rule of inference within the theory of natural numbers, in order to make sure it gives the results that are expected before applying it to new domains.
Additionally dualism embodies what I see as a very poor philosophical strategy, namely that of primarily attacking an opposing position and then claiming that the problems with that position give yours support (of course everyone does this to some extent, but most go on to argue for their theory itself by detailing its virtues and explanatory power). Dualists love to find flaws with materialism and then conclude that those flaws disprove materialism and thus prove dualism. But of course this is a silly strategy. First of all dualism is not the only alternative to materialism, and so even if we were to toss out materialism that would not force us to conclude that dualism was correct. And, more importantly, dualism has many flaws as well, as pointed out by materialists. If flaws with materialism are supposed to count against it than flaws with dualism should count against it as well. But dualists never seem to realize this, and seem to think that if they can just rule out materialism then dualism would be validated. This method, besides being relatively unconvincing, is simply a poor way to construct a theory, because less time is spent on the dualist theory of mind itself than is spent worrying about how that theory can be established and respond to objections (not to say that materialists never do this, but most of them have moved on, and focus more on what representational/functional properties are responsible for consciousness).