Full Idea
That there could be inhabitants of the moon, even though no human being has ever perceived them, must of course be admitted; but this means only that in the possible progress of experience we could encounter them.
Gist of Idea
There are possible inhabitants of the moon, but they are just possible experiences
Source
Immanuel Kant (Critique of Pure Reason [1781], B521/A493)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.512
A Reaction
This seems a fairly precise statement of phenomenalism (compare A.J. Ayer's Idea 5170). Kant calls himself a 'transcendental idealist', which seems something like a true idealist who acknowledges Humean 'natural beliefs' in reality.
Related Idea
Idea 5170 Material things are constructions from actual and possible occurrences of sense-contents [Ayer]