I worked for nineteen years at a Developemental Center. Our clients were often diagnosed as autistic. Since the people that I worked with were in their teens or were adults, I didn't get the opportunity to observe how autism manifests in infancy. However, I saw the eventual result.
    One young man ,who I will call Jimmy, was about sixteen when I first knew him. In many ways he was like a toddler who is still in the play alone or playing parallel to another child phase. At that stage a child has not started the social interaction with others that will become so marked in normal behavior by three or four year olds. Jimmy liked to run away from staff and would sit down and laugh sometimes when he was told to do something, however, most of the time he sat on the floor spinning anything and everything he could find and watching it go around and around. He seemed to turn his entire focus inward and was often unresponsive to anything outside himself. For the most part, Jimmy would not feed himself although he would chew and swallow when he was fed.
    Another man, who I will call Bobby, was unpredictably aggressive to anyone in his environment, although the other person was not talking to him or in any way interacting with him. He would just leap up and attack. It was as if the stimulus for his actions came from inside him and were not dependent on what was happening in the real world.. He would  also leap to his feet and suddenly run straight into a wall making no effort to stop himself or to protect himself from the blow. He entertained himself with painting his own feces all over his body untless closely supervised. He also liked to spin things or to watch any repetitively moving object. I have wondered if this focus on spinning objects might be some form of self hypnosis. He seemed to be involved almost entirely in his own world and to see other people as objects for manipulation. He could feed himself with a spoon, but for the most part his interaction with the world outside himself was practical only, as if he was the only person in the world and others just objects to be knocked over or shoved aside.
    I'm astounded at how lonely this must be.I'm told that often autistic infants do not like to be held. In fact they may find human contact to be unpleasant or painful, and so they turn away from what hurts them.
     In some forms of mental illness people become catatonic which  I believe is similar. All attention is focused on someplace deep inside the mind where no one else can go. I see it as floating in a meaningless universe all alone with no true reference to what those on the outside say, do, or beleive. I guess this is what the phrase "Being a world unto yourself" means.