  The feature of Kant's conception of natural science proper that is most immediately striking is how restrictive it is. It requires that cognition (i) be systematically ordered (ii) according to rational principles and (iii) be known a priori with apodictic certainty, i.e., with “consciousness of their necessity” (4:468). Because properly scientific cognition must satisfy these strict conditions, it requires “a pure part on which the apodictic certainty that reason seeks can be based” (4:469). But since Kant identifies pure rational cognition that is generated from concepts with metaphysics, it follows that science proper requires a metaphysics of nature. He then specifies that such a metaphysics of nature could consist in either a “transcendental part,” which discusses the laws that make possible the concept of a nature in general — “even without relation to any determinate object of experience” (4:469) — or a “special metaphysical” part, which concerns a “particular nature of this or that kind of things” for which an empirical concept is given.