Schwitzgebel argues by going through an elaborate and exhaustive taxonomy of Philosophy of Mind theories -- materialism, dualism, idealism, etc, showing that each in some way is both bizarre, ie it violates common sense in some way -- and dubious -- ie, it is not compelling enough to be accepted as true despite its oddity. The combination of these two qualities equals crazy in his terminology. The second clause about dubiosity is there to distinguish metaphysical craziness from scientific theories that, while bizarre enough, have evidence or something else compelling behind them. Eg, quantum mechanics is merely bizarre, because its strangeness can be supported by demonstration and experiment, while the many-worlds interpretation of qm is full-on crazy.
For instance, Schwitzgebel tries to demonstrate that a consistent materialism must grant consciousness to collective entities:
It would be bizarre to suppose that the United States has a stream of conscious experience distinct from the conscious experiences of the people who compose it...Yet it’s unclear by what materialist standard the United States lacks consciousness. Nations, it would seem, represent and self-represent. They respond (semi-) intelligently and self- protectively, in a coordinated way, to opportunities and threats. They gather, store, and manipulate information. They show skillful attunement to environmental inputs in warring and spying on each other. Their subparts (people and subgroups of people) are massively informationally interconnected and mutually dependent, including in incredibly fancy self-regulating feedback loops. These are the kinds of capacities and structures that materialists typically regard as the heart of mentality.Crazyism is inevitable, he says, because of the manifest inadequacy of common sense in dealing with metaphysics. "Something bizarre must be true about the mind, but which bizarre propositions are the true ones, we are in no good position to know." Crazyism is apparently an extension and partial one-upping of mysterianism, the idea that some phenomena (consciousness) are simply inexplicable due to the limitations of our minds. Mysterians are mostly naturalists, while Schwitzgebel is willing to entertain stranger ideas, and indeed believes one has to.
Common sense is incoherent in matters of metaphysics. Contradictions thus inevitably flow from it, and no coherent metaphysical system can respect it all. Although ordinary common sense serves us fairly well in practical maneuvers throughthe social and physical world, common sense has proven an unreliable guide in cosmology and probability theory and microphysics and neuroscience and macroeconomics and evolutionary biology and structural engineering and medicine and topology. If metaphysics more closely resembles items in the second class than in the first, as it seems to, we might excusably doubt the dependability of common sense as a guide to metaphysics.After living with this idea for awhile, the more sense it makes to me. The contrary position, that the universe is knowable by common sense down to its foundations, seems simplistic, arrogant, and counter to experience. And Schwitzgebel, in his clever attempts to apply reason at the point where reason breaks down, seems almost Godelian.
[Schwitzgebel has a blog, The Splintered Mind]