Refutation Of Idealism

Descartes inferred from his cogito argument that mind and body were separate in substance, which meant that the first could exist apart from the second. Bound up with this was the view that I am immediately aware of myself as a mind, but need to infer the existence of material things, which is in principle open to doubt. A great many philosophers have subscribed to this opinion, but Kant thought he could show it to be definitively false. In order to say that my inner experiences come one before another I need to observe them against a permanent background, and this can only be a background of external objects, for there is nothing permanent in the flow of inner experience. As Kant put it in the second edition [of Critique Of Pure Reason], in which he transposed the argument to the discussion of existence in connection with the postulates of empirical thought, "The mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me."
--W.H. Walsh, Kant, Immanuel, The Encyclopedia Of Philosophy (Collier-Macmillan)

I bolded what I think is the crucial passage. I'm not sure I understand either half of its argument, or why I should find the argument persuasive, though I don't have a clear objection to it either. Why does time need a background of permanence? And why can't inner experience contain anything permanent? And also, what's "external" and what's "internal"? While Descartes would say that initially (until we've gone through his own arguments) we can't know that the objects we observe are actually outside the mind (rather than being inventions or hallucinations or dreams etc.), Kant seems to be arguing that because the objects are permanent, they must be outside the mind; but that seems circular. Why can't you just say that some of what's "in" the mind can be "permanent"? And what does "permanent" mean here?

( Richard Rorty's The Invention Of The Mind )