Isn't it curious that infants find social goals easier to accomplish than physical goals, while adults find the social goals more difficult? One way to explain this is to say that the presence of helpful people simplifies the infant's social world — since because of them, simpler actions solve harder problems. Another explanation might be that the infant's social world is just as complicated as that of the adult, except the presence of helpful people makes the infant's mind more powerful — by making the agencies inside those other people's brains available for exploitation by the agencies in the infant's brain. Both explanations are the same, except for drawing different boundaries.
How do children start on the path toward distinguishing between psychological and physical relationships? In the appendix I'll suggest that our infant brains are genetically equipped with machinery for making it easy to learn social signals. But what if that machinery should somehow fail, so that by chance — or by neglect or accident — the realm-divisions never form? Then all those different kinds of thoughts would fuse together into one — and the child would face the impossible task of formulating principles that work in all domains. A child that tried to see the world without dividing it into realms would find no simple rules at all that work across so large a range.
This is why each child must learn different rules for the physical and psychological realms. But this means that the child must face not merely two formidable problems, but three. In addition to developing two different sets of concepts, the child must also develop agencies to manage those concepts by keeping them apart in different agencies, as we saw when we talked about Papert's principle.
This could explain some aspects of the disorders of the children psychiatrists call autistic. These unhappy individuals do not establish effective communication with other people, although they may acquire some competence at dealing with physical things. No one knows the causes of those disorders. Some might begin when certain mental realms do not develop normally. Other kinds of problems could emerge after those divisions form, if their separateness were compromised by some too intense attempt to unify them. To be sure, that is what scientists do, but unlike those whom we regard as mentally ill, scientists also manage to maintain their ordinary views. Once a child is deprived of the normal ways to divide those realms — no matter what the cause of this — that hapless mind is doomed to fail.