The third group of statements concerns the principle of pre-established harmony. "Is it possible," asks Kant, "that Leibniz meant by this doctrine to assert the mere coincidence of two substances wholly independent of each other by nature, and incapable through their own force of being brought into community?" And his answer is that what Leibniz really implied was not a harmony between independent things, but a harmony between modes of knowing, between sense on the one hand and understanding on the other. The "Critique of Pure Reason" carried the discussion farther by pointing out its grounds; namely, that, without the unity of sense and understanding, no experience would be possible. _Why_ there should be this harmony, _why_ we should have experience, this question it is impossible to answer, says Kant,--adding that Leibniz confessed as much when he called it a "pre-established" harmony, thus not explaining it, but only referring it to a highest cause. That Leibniz really means a harmony within intelligence, not a harmony of things by themselves, is made more clear, according to Kant, from the fact that it is applied also to the relation between the kingdom of nature and of grace, of final and of efficient causes. Here the harmony is clearly not between two independently existing _external things_, but between what flows from our notions of nature (_Naturbegriffe_) and of freedom (_Freiheitsbegriffe_); that is, between two distinct powers and principles _within us_,--an agreement which can be explained only through the idea of an intelligent cause of the world.