How can we distinguish one mind from another? How are we to determine what unifies the mind? Given radical mental disunity, these questions need to be answered.

Commissurotomy or split-brain patients may have two minds in one brain. Mind and consciousness may also split in self-deception and multiple-personality disorder. The author investigates these strange phenomena and considers the theories of classical philosophers like Hume and Kant in the light of current philosophical thinking.

Mind, consciousness and self are distinguished, and in working out criteria for the unity of the mind and explaining synchronic personal identity he puts forward a distinctive philosophy of mind. Real-life cases are considered as well as thought experiments. All these are subject to the realistic constraint that they could occur in a world where everything is supervenient upon some laws of nature