 Despite his warnings about the limited powers of human reason, Kant too believed it possible to deduce some features of matter, as physical science must theorize it, a priori. There are no material atoms; matter is divisible to infinity and its parts are all material (4:503f). Yet Kant recognized, first in the Physical Monadology, then in the Metaphysical Foundations, particles in the form of centres of attractive and repulsive forces that account for the space-filling property and impenetrability of matter (4: 533ff). This relatively dogmatic treatment co-exists with his critical claim that matter is the appearance of a perfectly unknown substratum. As he explains it in the Critique of Pure Reason, the rainbow is a mere appearance relative to rain drops which, in a physical sense, are things-in-themselves and not mirages. Yet thinking further, we realize that the raindrops too are mere appearances, and that “even their round form, indeed, even the space through which they fall are nothing in themselves, but only mere modifications or foundations of our sensible intuition; the transcendental object, however, remains unknown to us.” (KRV A 45f/B 63f). “About these appearances, further, much may be said a priori that concerns their form but nothing whatsoever about the things in themselves that may ground them.” (KRV A49/B66). This suggests that the stuff which is divisible to infinity and bears attractive and repulsive forces is an appearance of something unknown and unknowable. “We can understand nothing except what brings with it something in intuition corresponding to our words. When we complain that we do not see into the inner nature of things, this can mean no more than that we cannot grasp, through pure reason, what the things that appear to us might be in themselves…. Observation and division with respect to the appearances take us into the interior of nature, and we cannot say how far this will proceed. But every transcendental question that takes us beyond [perceptible] nature can never be answered…” (KRV (A277f/B333f).