There is a further argument for substance dualism which rests on the impossibility of vagueness in whether a certain person would have been me if certain things had been different at my conception. The point is that the sort of vagueness in origin, giving rise to mere overlap with no strict fact about identity cannot be applied in the subjective case. This case differs from the problem of identity through time because that rests on the presence of qualitative difference. I might imagine a person with exactly the same history as myself, but minor differences in origin, so there is no empathetic distance between him and me, but the notion of subjective overlap – parallel to the overlap of constitution in the physical cases, is impossible to make sense of. So the self is a genuine individual and unity in the sense in which no physical object is. I also try to see how a simple entity of this sort could also be the complex phenomenon that is a human mind. I appeal to Geach’s account of thought.