Philosophy, then, according to Kant, is to abate its claims. It is warned off the premises of everything except immediate existence in space and time. It must give up all attempts to know reality, to penetrate behind appearances. But the effect of this solemn warning upon the philosophic world was truly astonishing. No sooner had Kant thus cried “Halt!” to philosophy than philosophy, forming its adherents into a sort of triumphal procession, proceeding, so to speak, with bands playing and flags waving, marched victoriously onward to the final assault, confident of its power to attain omniscience at a stroke, to occupy the very citadel of reality itself. And, strangest of all, this was to be done with the very weapons which Kant himself had forged. It was under the Kantian banner that philosophy moved forward. It was Kant’s own philosophy, hailed as the greatest discovery of all time, which was to accomplish this final and triumphant victory. Philosophy, instead of being sobered by the warnings of the master, rose at once to an exuberant ferment of enthusiasm. It set no bounds to itself. It was to accomplish the impossible, know the unknowable. Such is the confident enthusiasm of the philosophies of that time.
And one more interesting remark from that book (p.76)…
This idea [that there is an absolute separation between things and thoughts] was originated by Descartes and dominates modern philosophy until it culminates in Kant, whose philosophy is nothing else than the reductio ad absurdum of it.
Here the reductio that Stace has on mind is the notion of thing-in-itself, which on one side is supposed to be something which can’t fall under the categories, but which on other hand is described in the Critique as standing in the relation of cause and effect with the sensuous content which appears in the forms of space and time.