The Stoic philosopher Chrysippus (c. 280 B.C.–c. 206 B.C.) is said to have posed the following puzzle. Yesterday, there was a whole bodied man called 'Dion' who had a proper part called 'Theon'. Theon was that part of Dion which consisted of all of Dion except his left foot. Today, Dion's left foot was successfully amputated. So, if Dion and Theon both still exist, they are numerically different objects now occupying the same place and wholly composed of just the same matter. Presuming this to be impossible, the question is which of the two, Dion or Theon, has ceased to exist.[1]

At first thought, of course, it seems that neither has ceased to exist. It would seem absurd to deny that Dion is still with us. Surely, a man can retain his identity despite the loss of a foot. But it also seems undeniable that Theon still exists. Theon, it seems, has emerged from the surgery intact.

Might it be that Dion and Theon, who initially were two, have both survived, but now are one? Assuming the indiscernibility of identicals, a principle invoked even in Hellenistic philosophy, the answer is “no.” For even now there is something true of Dion which is not true of Theon: that he once had two feet.

Michael B. Burke, Dion and Theon.

[1]Note the assumption that Dion is wholly material, without which there would be no significant problem about Dion's occupying the same place as Theon. Those who reject the assumption (i.e. dualists) can let 'Dion' name a bird, a doll, or a human body.