The section is interesting because Kant also argues there for a form of verificationism: namely, that, for a large class of states of affairs, if they obtain, they can be known to obtain. The example Kant gives concerns lunar inhabitants, but it seems clear that his argument is meant to generalize quite broadly:
That there may be inhabitants in the moon, although no one has ever observed them, must certainly be admitted; but this assertion means only, that we may in the possible progress of experience discover them at some future time.In the bold part, Kant states:
"there may be inhabitants in the moon" means only that we may in the possible progress of experience discover them at some future time.Here Kant uses a modal qualifier (translated as "may"). This makes it a little more difficult to analyse. Still, in general, for sentences "p" and "q" of English, the scheme
If "$p$" means only that $q$, then if $@_w, p$, then $@_w, q$.holds. For example,
If "$5+7 = 12$" means only that, whenever there are 5 Fs, 7Gs and no Fs are Gs, then there are 12 F-or-Gs, then if $@_w, 5+7 = 12$, then $@_w$, whenever there are 5 Fs, 7Gs and no Fs are Gs, then there are 12 F-or-Gs.So, Kant's "means only that" claim implies:
If, at $w$, there are inhabitants in the moon, then at $w$, they will be discovered, in the progress of experience.Because the modal operator $@_w$ commutes with logical connectives, this implies:
At $w$: if there are inhabitants in the moon, then they will be discovered, in the progress of experience.Hence, Kant's claim implies the following:
(Moon) If there are inhabitants in the moon, then they will be discovered, in the progress of experience.This is an instance of the more general verificationist principle,
(VP) If p, then it can be known that p.And Kant thinks that (VP) --- cf., the claim (Moon) that moon-dwellers are detectable --- holds of necessity. A claim such as (VP) is an epistemic access claim, a form of verificationism or epistemological reductionism, akin to modern Dummettian anti-realism: the view that all truths (of some class) are knowable.
But why should this be so?
Suppose, hypothetically, that there are inhabitants of the moon. How does one infer that they will be discovered, in the progress of experience? Why might they not be microscopic, too small to be seen by the human visual system? Why might they not perhaps vanish, whenever anyone looks for them? Why might they have momentary existence, with timelines of one millisecond in duration? And so on.