Immanuel Kant's Critique of Pure Reason, published in 1781, did not add a new metaphysical system to the ones already on offer, it argued that the entire prior project, reasoning from pure concepts alone to conclusions about God, the soul and the ultimate structure of reality, had been asking a question the human mind is not equipped to answer. Kant proposed what he called a Copernican revolution in reverse: rather than assuming the mind must conform to objects in order to know them, he argued that objects, as we can ever experience them, must conform to the mind's own built in structures of space, time and causal reasoning, so that human knowledge reaches only appearances, phenomena shaped by those structures, and never things as they are in themselves, noumena, independent of any observer.
The consequence for metaphysics as it had been practiced by rationalists including Leibniz and Christian Wolff was severe. Traditional metaphysical questions about whether the universe had a beginning in time, whether the soul is a simple indestructible substance, whether a necessary being exists, produced, Kant argued, equally valid seeming arguments on both sides, antinomies that proved reason had overreached the only domain, possible experience, in which it can settle anything. Metaphysics after Kant split along the fault line his critique opened: some schools abandoned the old rationalist ambitions entirely, while others, notably German Idealism, tried to rebuild metaphysics on Kant's own insight that the structure of experience itself was the proper subject of inquiry rather than a distraction from it.