Non-Cartesian Substance Dualism

Perhaps. He seems to be saying in that quote that if the self can't be externally perceived, then for all we know it's rapidly being replaced moment to moment. Why only external perception counts would then perhaps be explained as you go on to suggest: that during periods of unconsciousness like (supposedly) sleep, internal perception ceases.

Except that we know that internal perception doesn't cease ~ it's just different. The fact that we can have dreams that we don't even recall, only for memories of said dreams being triggered later by something in the waking world is evidence of this.

For me, the argument's a non-starter even allowing for periods of unconsciousness. Why would we entertain the bizarre possibility that our selves might replace themselves from moment to moment in the first place, whether conscious or unconscious? Even if we did entertain it as a possibility, why would we then take it seriously enough to consider it the basis of an argument against substance dualism? It makes no sense to me.

I don't think it's logical in any case ~ there's no evidence for it, and it's one of the poorer arguments against Substance Dualism.