Cheever gives us suggestions that he is truly out of touch with reality and has been so for quite a while. He returns to his home after his swim across the county and finds the house empty---not just of people but of everything. It is unlikely that his wife and daughters moved out in the time it took him to swim the county, so we must assume they have been gone for some time and that Neddy has lost his sanity. A rain gutter is down, there is rust on the garage door handles:
He shouted, pounded on the door, tried to force it with his shoulder, and then, looking in at the windows, saw that the place was empty (Cheever 445).
Cheever is certainly not telling the reader what it means to be an individual human being, but he is telling us what it means to go mad as a result of being an inauthentic human being. Neddy is a man who has lived a self-centered life, doing what he wanted to do, perhaps simply pursuing the superficial materialistic goals society has instilled in him. In any case, he apparently has no idea what others truly think of him, or what they are feeling. He has paid the price of his sanity, at the very least, for his superficial and selfish life.
Lewis Thomas in his protest against the cloning of human beings also has more to say about what is not authentic human reality than what is:
The cloning of humans is on most of the lists of things to worry about from Science, along