To see the mistake we need to recognize another mistake Descartes made: his denial that other animals have any mental lives at all. Careful field observation by primatologists beginning with Jane Goodall revealed that apes have well developed “theories of mind.” They engage in “mind reading” to make (sometimes good) guesses about the future behavior of others. Mind reading is psychologists’ shorthand for treating other animals as having something like desires and beliefs that work together to produce choices in behavior.

After a certain point in the evolutionary past, organisms began needing to predict whether others posed threats in order to protect themselves, and later needed to coordinate to attain outcomes not achievable alone. This environment strongly selected for mind reading. Had variation in cognitive abilities not hit on this adaptation, puny creatures like us would never have survived in the face of savanna megafauna.

Mind reading, even in our own hands, is a very imperfect tool: We have to go on others’ behavior (including verbal behavior). We can’t really tell with much precision exactly what others believe or want, because we can’t get inside their heads. So our predictions are often pretty vague and frequently false. Like other Darwinian adaptations, mind reading is an imperfect, “quick and dirty” solution to a “design problem.” It was just good enough that, equipped with this theory of mind, we managed to gradually climb to the top of the food chain. We were able to do so in large part because once mind reading was in place human language, which requires it, became possible.

FMRI research, the study of autism, and experiments on infant “false-belief” detection have shown that mind reading is a relatively well-localized module in the human brain, innate in structure, subject to breakdown — often genetically caused, and identifiable in infant/toddler development.

Most important, there is compelling evidence that our own self-awareness is actually just this same mind reading ability, turned around and employed on our own mind, with all the fallibility, speculation, and lack of direct evidence that bedevils mind reading as a tool for guessing at the thought and behavior of others. When, as David Hume said, we look into ourselves, all we ever see are images, all we ever hear is silent speech-sounds. These sensations (along with emotions) are the only contents of consciousness, the only things introspection can use to figure out what we are thinking. The resources of introspection are exactly the same as the resources our minds work with to explain and predict the actions of others: sensory data provided by sight, hearing, smell, touch (and sometimes taste, too).

Of course we have a lot more sensory data — images and silent speech instead of visual experience and heard speech — to go on in trying to figure out our own desires and beliefs than what other people’s behavior reveals about what is going on in their minds. That’s part of what makes for the illusion that we know our own minds so much better. But the difference is only the amount of data, not its quality or source. We never have direct access to our thoughts. As Peter Carruthers first argued, self-consciousness is just mind reading turned inward.

How do we know this? Well, Hume would have answered that introspection tells us so. But that won’t wash for experimental scientists. They demand evidence. Some of it comes from the fMRI work that established the existence of a distinct mind-reading module, more from autistic children, whose deficits in explaining and predicting the behavior of others come together with limitations on self-awareness and self-reporting of their own motivations. Patients suffering from schizophrenia manifest deficiencies in both other-mind reading and self-mind reading. If these two capacities were distinct one would expect at least some autistic children and schizophrenics to manifest one of these capacities without the other.