To reach this limit by empirical means is impossible, and all attempts to determine it "a priori" according to a principle, are alike in vain. But all the questions raised by pure reason relate to that which lies beyond this horizon or, at lease, in its boundary line.
The celebrated David Hume was one of those geographers of human reason who believe that they have given a sufficient answer to all such questions, by declaring them to lie beyond the horizon of our knowledgeĐa horizon which, however, Hume was unable to determine.
(Immanuel Kant "Critique of Pure Reason" Translated by J. M. D. Meiklejohn)