The Amoeba Split

The case

An amoeba divides. Call the original A, and the two resulting cells B and C. Each daughter cell is biologically continuous with A: each inherited A's genetic material, each developed from A's physical substance, each stands in the same causal relationship to A.

Now ask: is B identical to A? It seems it should be, given the continuity. Is C identical to A? The same reasoning applies. But B and C are not identical to each other. They are two distinct organisms, spatially separate, doing different things from the moment of division. Standard logic says if B equals A and C equals A, then B equals C. But B does not equal C. Something has gone wrong.

The problem for identity theories

The amoeba split is a fission case, and fission cases expose a structural problem for any theory that analyzes identity purely in terms of continuity. Continuity, whether physical, biological, psychological, or causal, is a relation that can branch. Identity cannot. Leibniz's Law requires that if X and Y are identical, they share all properties. B and C do not share all properties after the split, so at most one of them can be identical to A.

But which one? Nothing in the situation distinguishes them. Any reason to say B is the "real" original would apply equally to C. The most natural conclusion is that neither is strictly identical to A, even though each is continuous with A. This means that continuity, even perfect physical continuity, is not sufficient for identity.

Wiggins's conclusion

Wiggins drew a conservative lesson: genuine identity requires stricter criteria than mere continuity. For biological organisms, he argued for something like sortal-relative identity: what it is for an organism to persist depends on what kind of thing it is, and the persistence conditions of different kinds of things differ. An amoeba that divides does not survive as a numerically identical amoeba; two new amoebas come into existence.

This connects to Parfit's fission problem for persons: if both hemispheres of your brain were transplanted into different bodies, each recipient would be continuous with you. Wiggins's amoeba analysis suggests that in such a case, you would not survive at all, even though something continuous with you does. What survives is not you but something that inherits your continuity. Parfit thought this was the right answer, and that it should change how we think about the importance of personal identity.