The Physicist is led by observation of the phenomena which occur in our environment when, e.g., we experience a sensation of sound, to the view that there is nothing in these phenomena which in the very least resembles what we perceive immediately as sound. Outside, in the space which surrounds us, nothing is to be found except longitudinal oscillations of bodies and of the air. Thence it is inferred that what in ordinary life we call "sound" or "tone" is nothing but the subjective reaction of our organism to these wave-like oscillations. Similarly, it is inferred that light and colour and heat are purely subjective. The phenomena of colour-dispersion, of refraction, of interference, of polarisation, teach us that to the just-mentioned sensations there correspond in the outer space certain transverse oscillations which we feel compelled to ascribe, in part to the bodies, in part to an immeasurably fine, elastic fluid, the "ether." Further, the Physicist is driven by certain phenomena in the world of bodies to abandon the belief in the continuity of objects in space, and to analyse them into systems of exceedingly minute particles (molecules, atoms), the size of which, relatively to the distances between them, is immeasurably small. Thence it is inferred that all action of bodies on each other is across the empty intervening space, and is thus a genuine actio in distans. The Physicist believes himself justified in holding that the action of bodies on our senses of touch and temperature does not take place through direct contact, because there must always remain a definite, if small, distance between the body and the spot on the skin which it is said to "touch." Thence it is said to follow that what we sense as hardness or heat in bodies is nothing but the reactions of the end-organs of our touch- and temperature-nerves to the molecular forces of bodies which act upon them across empty space. These considerations from the sphere of Physics are supplemented by the Psycho-physicists with their doctrine of specific sense-energies. J. Müller has shown that every sense can be affected only in its own characteristic way as determined by its organisation, and that its reaction is always of the same kind whatever may be the external stimulus. If the optical nerve is stimulated, light-sensations are experienced by us regardless of whether the stimulus was pressure, or an electric current, or light. On the other hand, the same external phenomena produce quite different sensations according as they are perceived by different senses. From these facts the inference has been drawn that there occurs only one sort of phenomenon in the external world, viz., motions, and that the variety of qualities of the world we perceive is essentially a reaction of our senses to these motions. According to this view, we do not perceive the external world as such, but only the subjective sensations which it evokes in us.