A little boy sits at the top of the slide, oblivious to the restless queue snapping behind him. He stares into space . and sings. In a hardware store, he plonks himself on a display toilet amidst the throng of customers and wees, wearing little more than a serene smile. He thumps crying babies. He is amazed, as he hurtles skyward, when the car he runs in front of actually hits him. Joe is ten and mentally disabled. He exists in a lonely bubble of misunderstanding and occasional calamity.

He's funny, fascinating and maddening. He has strange, unexpected talents and his life is a catalogue of the bizarre. The book tells his moving story, but it also argues something more, something brazen, preposterous even: that Joe is a profound lesson in our own humanity, and that until we get to know his unusual, eventful life, we can't fully understand our own.Drawing on philosophy, developmental psychology, evolutionary theory, literature and medical research, the thought-provoking inquiry shows how we, in contrast, to Joe, are equipped to negotiate life. The Only Boy in the World, by his oddity and isolation, makes luminous so much that we take for granted: how we are instinctive mind-readers, how we perceive the physical world around us, how we make sense of other people, how we understand guilt and innocence. Running throughout is an unsettling question: if Joe sets our humanity in such sharp relief, can we still say that he is part of it? If he shows by what he lacks all that is most remarkable in us, is he still one of us? The author who asks that outrageous question is Joe's father. In confronting it, the Only Boy in the World finally suggests an answer to what truly makes us human.