William Lycan is a materialist philosopher, i.e. he rejects the dualist view that there are two fundamental essences to reality - matter and mind. He believes that matter is all there is and that mind is just a word we use to describe the functioning of the brain.
Nevertheless, in a paper on the subject he acknowledges that, though it pains him to say it, materialism is little more than a prejudice. The arguments for materialism, he notes, are no better than the arguments for dualism:
I mean to have shown here that although Cartesian dualism faces some serious objections, that does not distinguish it from other philosophical theories, and the objections are not an order of magnitude worse than those confronting materialism in particular. There remain the implausibilities required by the Cartesian view; but bare claim of implausibility is not argument. Nor have we seen any good argument for materialism. The dialectical upshot is that, on points, and going just by actual arguments as opposed to appeals to decency and what good guys believe, materialism is not significantly better supported than dualism.
Yet, I am inclined to believe, the charge of implausibility is not irrational or arational either, and I would not want this paper to turn anyone dualist. Have a nice day.
In the paper Lycan observes that the strongest argument against dualism is the incomprehensibility of two fundamentally disparate substances, mind and matter, interacting in the brain to produce mental phenomena. It's hard to imagine how an immaterial substance like mind could act causally on matter. Lycan doesn't think that this is much of an objection because we scarcely understand causality in the first place.In this he is joined by a lot of other philosophers who, in the words of J.P. Moreland, find the so-called interaction problem to be "the most exaggerated problem in the history of philosophy." I discussed the problem recently here.
If the materialist wishes to cite "interactionism" as an objection to dualism he must confront the inconvenient difficulty that materialists themselves have long believed that disparate entities or substances could interact even though their interaction was incomprehensible. How, for example, does space generate quantum particles? Indeed, how does matter bend space? Whatever one's conception of space it's very difficult to conceive how such things happen. If a materialist, nevertheless, believes they do, as most thinkers since Einstein have, then it seems a case of special pleading to exclude mind/brain interaction on the grounds that we have no theory to explain how it could occur.