What does the sentence in boldface mean? I'm confused about the unless in this sentence.
WEIROB: Well, consider these possibilities. One is that a single soul, one and the same, has been with this body I call mine since it was born. The other is that one soul was associated with it until five years ago and then another, psychologically similar, inheriting all the old memories and beliefs, took over. A third hypothesis is that every five years a new soul takes over. A fourth is that every five minutes a new soul takes over. The most radical is that there is a constant flow of souls through this body, each psychologically similar to the preceding, as there is a constant flow of water molecules down the Blue. What evidence do I have that the first hypothesis, the “single soul hypothesis” is true, and not one of the others? Because I am the same person I was five minutes or five years ago? But the issue in question is simply whether from sameness of person, which isn’t in doubt, we can infer sameness of soul. Sameness of body? But how do I establish a stable relationship between soul and body? Sameness of thoughts and sensations? But they are in constant flux. By the nature of the case, if the soul cannot be observed, it cannot be observed to be the same. Indeed, no sense has ever been assigned to the phrase “same soul.” Nor could any sense be attached to it! One would have to say what a single soul looked like or felt like, how an encounter with a single soul at different times differed from encounters with different souls. But this can hardly be done, since a soul according to your conception doesn’t look or feel like anything at all. And so of course “souls” can afford no principle of identity. And so they cannot be used to bridge the gulf between my existence now and my existence in the hereafter.
MILLER: Do you doubt the existence of your own soul?
WEIROB: I haven’t based my argument on there being no immaterial souls of the sort you describe, but merely on their total irrelevance to questions of personal identity, and so to questions of personal survival. I do indeed harbor grave doubts whether there are any immaterial souls of the sort to which you appeal. Can we have a notion of a soul unless we have a notion of the same soul? But I hope you do not think that means I doubt my own existence. I think I lie here, overweight and conscious. I think you can see me, not just some outer wrapping, for I think I am just a live human body. But that is not the basis of my argument. I give you these souls. I merely observe that they can by their nature provide no principle of personal identity.