I wish to make a very general point about the complexity of the mind—not just the human mind but minds generally. There has been a tendency to think that the mind is relatively simple: it consists of simple ideas and principles of combination of these ideas; or it is a collection of reflexes, conditioned and unconditioned; or it is basically a blank slate; or it is composed of beliefs and desires; or it is made up of behavioral dispositions; or it consists of algorithms that are reducible to operations on ones and zeroes. At a more immediate level, introspection doesn’t reveal a particularly complex entity: our consciousness may be puzzling theoretically, but it is not all that complex superficially. Compare it to the body and brain: everyone with any knowledge of the subject knows that the body and brain are amazingly complex entities, far more complex that casual inspection reveals. If you just look at a human body, you observe some complexity, yes, but not the enormous complexity that exists beneath the skin. This is why people were surprised to discover the degree of complexity hidden in the animal body: all those varying organs, the cellular structure, the interior of the cell, the packed intricacy of the brain. Commonsense does not anticipate the discoveries of anatomy and physiology, which reveal a complexity that goes far beyond what we might have imagined. No doubt all this complexity has a biological rationale; it is not to be supposed that evolution would equip the body with so much complexity for no reason. This is functional complexity not gratuitous ornamentation. But we don’t tend to adopt the same view of the mind: we don’t tend to think that the mind is vastly more complex than it appears. We tend to think that its complexity is limited to what we observe, give or take a bit. True, Freud supposed that there is a substantial hidden portion to the mind, viz. the unconscious; and modern proponents of an unconscious mind have likewise posited some underlying complexity. But I don’t think the magnitude of the mind’s hidden complexity has been compared to the magnitude of the body’s hidden complexity, as these have been conceived. Does the mind have the same degree of hidden complexity that the body has? Is it, in particular, as complex as the brain? I think it has been assumed that the answer to these questions is No—because it is not supposed that the mind requires so much hidden machinery.
Why is this? Evidently it’s because we don’t know as much about the hidden complexity of the mind. No doubt before anatomy and physiology developed (not to mention molecular biology) people underestimated the complexity of the body, supposing it to be not to be much more complex than it appears to the naked eye. But science disabused them of that impression: it revealed the body to be a lot more complex than it seemed. Similarly, we now don’t know that the mind has this kind of hidden complexity, so we don’t tend to acknowledge it—why postulate complexity when there is no empirical need to? Maybe the mind is actually fairly simple, just as it appears. The mind is just one organ possessed by the animal, it may be said, so its complexity might not be greater than that of other organs, say the kidneys. Maybe the mind only uses part of the complex brain, so it lacks the complexity of the full brain. There is no principled reason that the mind should be more complex than it has appeared to us hitherto—medium-level complex, one might say. Isn’t the mind of an insect a good deal less complex than its body—as complex as its digestive system perhaps? Insects have simple minds, reptiles less simple minds, and mammals have minds that are still relatively simple in comparison with their bodies. External observation does not contradict this idea, and introspection confirms it. Hence the relatively simple theories of mind offered by behaviorism and empiricism.
The point I want to make is that this is unlikely to be true. It is more likely that the mind, like the body, harbors a vast inner complexity of which we have but a tiny inkling. That is, our impression of the mind’s lack of complexity results purely from our ignorance of its workings, rather as scientifically primitive people underestimated the body’s complexity for similar reasons. I don’t claim that this must be so, but it seems to me the more likely hypothesis. Why? The reason is that the mind, like the body, is the result of millions of years of assiduous evolution, constantly expanded, fine-tuned, minutely engineered, optimized, refined, and powered up. This is precisely why the brain is so complex—so as to cater to the needs of the mind. Of course, we don’t see all this on the surface—any more than we see the body’s complex machinery through the skin. Evolution has no need to show us its handiwork, but it does have a need to do that work—and it has been doing it for a very long time. It has been shaping, constructing, experimenting, honing, inventing, and advancing—just as it has with the bodies of animals. How could this not be so? The mind is bound to be complex—and just as bound not to be visibly so. Evolution is a relentless complexity-generator. We get a sense of this with recent work on vision in which the visual system emerges as much more complex than it seems to the average viewer (the complexity of visual constancy is notorious, despite its seeming simplicity). But much the same must be true of other mental phenomena, such as pain: pain is not some punctate atom with no inner complexity and an on-off etiology; it is a carefully calibrated biological system that has evolved over millions of years. In all likelihood the mind as a whole is an incredibly complex operation far exceeding what we currently imagine. It is just that so far we have received only hints of that complexity. And this is so for a rather obvious reason: we can’t open up the mind and have a peek inside, as we can the body. Dissection revealed the body’s hidden complexity (some of it), but dissection is no help in revealing the mind’s hidden complexity. For that we need theory, but psychology has not produced the kinds of deep theory that we might hope for; it hasn’t done for the mind what anatomy and physiology did for the body.
This perspective has an obvious consequence: an enormous proportion of the mind is not conscious. How much? Oh, let’s say 90% just to have a concrete figure. How much of the body’s complexity is hidden under the skin? I’d say roughly 90%, wouldn’t you? Most of the body is hidden from casual inspection: so isn’t it reasonable to expect that the same is true of the mind? Thus about 90% of the mind is unconscious. This estimate might be modified by advances in psychology, but it seems like a reasonable shot in the dark. If the mind has substantial hidden complexity, which general considerations about evolution suggest, then it follows that it has a substantial unconscious component. If the complexity were conscious, it wouldn’t be hidden to us; so given that it is hidden, it must be unconscious. We might picture this unconscious as analogous to the ceaseless operations of the cells of our body as they go about their life-sustaining business; likewise the machinery of mind hums away behind the scenes allowing us to function as we do and have the conscious minds we have. Language is a prime example here: there is good reason to believe that linguistic competence and performance are substantially organized unconsciously; and there may be deeper linguistic levels than any so far plumbed. The entire conscious mind is likely to be sitting on top of a mountain of unconscious machinery of unimaginable complexity, of which we are barely aware. We are like anatomists in the Stone Age, scarcely glimpsing the complexities of the human body. To be sure, our minds seem pretty simple to us now, but that may be an illusion born of ignorance. On God’s complexity-meter the mind might be the most complex thing in existence, despite its relatively simple façade. Not more complex than the brain obviously, but close to it. The brain contains billions of neurons and even more billions of neural connections: how many parts and part connections does the mind have? Maybe a million times what it now appears to have?
 I might also cite Wittgenstein as an apostle of psychological simplicity, given his doctrine that “nothing is hidden”. Granted he thinks that our language games can be quite complex, but the general tendency of his thought is to downplay any underlying complexity in our minds. It is true that he sometimes criticizes earlier philosophers and psychologists for oversimplifying the variety of mental phenomena, but he still keeps to a relatively simple view of the mind compared to the complexities of the body. (Note: this comment is subject to the usual caveats about interpreting Wittgenstein.)
 I should emphasize that judgments of simplicity are always relative, so when I say the mind seems pretty simple to us now I don’t mean simple simpliciter; I mean simple compared to things we regard as truly complex, like bodies and brains. Minds are certainly complex, according to common sense, compared to (say) leaves or electrons or grains of sand.
 The panpsychist will presumably concur, since he views the mind as a complex consisting of billions of mental elements corresponding to neurons (and their parts). For the panpsychist it is axiomatic that the mind is vastly more complex than it seems. Actually the same could be said of materialism, which might be used as an argument against materialism by an adherent of common sense: for how could the simple character of mental states consist in the kind of complex physical state present in the brain? Isn’t the feeling of pain a lot simpler than the physical state of C-fibers firing? The self indeed has been claimed to be simple simpliciter, whereas the body is extremely complex—so the former can’t be the latter. In any case, materialism confers the complexity of the body and brain squarely on the mind. The materialist is therefore committed to the idea that the mind has vastly more complexity than appears to us. I am suggesting the same picture but without the materialist assumption: the complexity of the mind is a mental complexity.