_a_) The mind-body problem becomes an integral part of the problem of knowledge. Sense organs, neurones, and neuronic connections are certainly involved in the occurrence of a sense quality. If the occurrence of the latter is in and of itself a mode of knowledge, it becomes a matter of utmost importance to determine just how the sense organs take part in it. If one is an idealist he responds with joy to any intimation that the "process of apprehension" (that is, speaking truly, the physical conditions of the occurrence of the sensory datum) transforms the extra-organic stimulus: the alteration is testimony somehow to the constitutive nature of mind! But if he is a realist he conceives himself under obligation to show that the external stimulus is transmitted without any alteration and is apprehended just as it is; color must be shown to be simply, after all, a compacting of vibrations--or else the validity of knowledge is impugned! Recognize that knowledge is something _about_ the color, whether about its conditions or causes or consequences or whatever, and that we don't have to identify color itself with a mode of knowing, and the situation changes. We know a color when we understand, just as we know a thunder-storm when we understand. More generally speaking, the relation of brain-change to consciousness is thought to be an essential part of the problem of knowledge. But if the brain is involved in knowing simply as part of the mechanism of acting, as the mechanism for co-ordinating partial and competing stimuli into a single scheme of response, as part of the mechanism of actual experimental inquiry, there is no miracle about the participation of the brain in knowing. One might as well make a problem of the fact that it takes a hammer to drive a nail and takes a hand to hold the hammer as to make a problem out of the fact that it also requires a physical structure to discover and to adapt the particular acts of holding and striking which are needed.