The theory of the so-called Specific Nervous Energy, which has been advanced by J. Müller, supplements these speculations. It asserts that each sense has the peculiarity that it reacts to all external stimuli in only one definite way. If the optic nerve is stimulated, light sensations result, irrespective of whether the stimulation is due to what we call light, or to mechanical pressure, or an electrical current. On the other hand, the same external stimulus applied to different senses gives rise to different sensations. The conclusion from these facts seems to be, that our sense-organs can give us knowledge only of what occurs in themselves, but not of the external world. They determine our percepts, each according to its own nature. Physiology shows, further, that there can be no direct knowledge even of the effects which objects produce on our sense-organs. Through his study of the processes which occur in our own bodies, the physiologist finds that, even in the sense-organs, the effects of the external process are modified in the most diverse ways. We can see this most clearly in the case of eye and ear. Both are very complicated organs which modify the external stimulus considerably, before they conduct it to the corresponding nerve. From the peripheral end of the nerve the modified stimulus is then conducted to the brain. Here the central organs must in turn be stimulated. The conclusion is, therefore, drawn that the external process undergoes a series of transformations before it reaches consciousness. The brain processes are connected by so many intermediate links with the external stimuli, that any similarity between them is out of the question. What the brain ultimately transmits to the soul is neither external processes, nor processes in the sense-organs, but only such as occur in the brain. But even these are not apprehended immediately by the soul. What we finally have in consciousness are not brain processes at all, but sensations. My sensation of red has absolutely no similarity with the process which occurs in the brain when I sense red. The sensation, again, occurs as an effect in the mind, and the brain process is only its cause. This is why Hartmann (Grundproblem der Erkenntnistheorie, p. 37) says, "What the subject experiences is therefore only modifications of his own psychical states and nothing else." However, when I have sensations, they are very far as yet from being grouped in those complexes which I perceive as "things." Only single sensations can be transmitted to me by the brain. The sensations of hardness and softness are transmitted to me by the organ of touch, those of colour and light by the organ of sight. Yet all these are found united in one object. This unification must, therefore, be brought about by the soul itself; that is, the soul constructs things out of the separate sensations which the brain conveys to it. My brain conveys to me singly, and by