No branching rule argument for multiplicity of self

Dr Shelly Kagan makes the argument in his Yale course Death Phil 176, lecture 13, http://oyc.yale.edu/philosophy/phil-176/lecture-13 that to have a singular personal identity one has to make a "no branching rule" for each of the three personal identity theories he proposes. The assumption is the self has to be singular --- there can not be more than one you at a time. To make this true every theory he proposes has to have a irrational clause that which he calls the "no branching rule". There is no way to scientifically prove that people can not branch into multiple bodies, personalities, or souls, so this has to become a founding axion of personality theory with no experimental proof. But is this axion true or is it just a desired belief like the parallel axiom in geometry, or that space and time does not bend or change speeds as proposed by Newtonian physics.

Dr Kagan makes the argument that we are either a body, a personality, or a soul. We have no scientific evidence that souls exist So we will eliminate that theory even though many people still believe it. But why can't we be both the multiplicity of bodies and personalities as superimmortality predicts?

The no branching rule is basically means that even though the body of a person can be divided so that more than one person or body could claim ownership of the self, it will not be allowed. The same goes for personality. There is nothing that does not allow two different people to have the same personality. If you are a personality, and two people have the same personality, then are you two people?

Open Yale courses | Death Phil 176

http://oyc.yale.edu/philosophy/phil-176/lecture-13 no divergence, no multiplicity of self, no branching rule

Lecture 13 - Personal Identity, Part IV: What Matters?