In man and all the higher animals the stimuli are received by the organs of sense and conducted by their nerves to the central organ. In the brain they are either converted into specific sensations in the sense-centres, or conveyed to the motor region, where they provoke movements. The conduction of stimuli is simpler in the lower animals and the plants; the tissue-cells either directly affect each other or are connected by fine threads of plasm. In the unicellular protists the stimulus which strikes one particular spot of the surface may be immediately communicated to the other parts of the unified plasmic body. We shall see in the course of our inquiry that the simplest form of sensation (in the widest sense) is common to inorganic and organic bodies, and thus that sensitiveness is really a fundamental property of all matter, or, more correctly, all substance. We may, therefore, ascribe sensation to the constituent atoms of matter. This fundamental thought of hylozoism, expressed long ago by Empedocles, has lately been very definitely urged, especially by Fechner. However, the able founder of psychophysics (_cf._ the _Riddle_, p. 35) assumes that consciousness (or thought, in the Spinozistic sense) always accompanies this universal property of sensation. In my opinion, consciousness is a secondary psychic function, only found in man and the higher animals, and bound up with the centralization of the nervous system. Hence it is better to speak of the unconscious sensation of the atoms as _feeling_ (_æsthesis_), and their unconscious will as _inclination_ (_tropesis_). It finds expression in the one-sided action of a stimulus as a "directed movement" or "stimulated movement" (_tropismus_ or _taxis_). The familiar ideas of sensation and feeling are often confused, and employed in very different ways in both physiology and psychology. The metaphysical tendency which so completely separates the two sciences, and the physiological tendency which agrees with it, regard feeling as a purely psychic or spiritual function, whereas in the case of sensation they have to admit the connection with bodily functions, especially sense-action. In my opinion, the two ideas are purely physiological and cannot be sharply separated, or only in the sense that sensation relates more to the external (objective) part of the sensory nerve-process, and feeling to the internal (subjective) part. Hence we may define the difference in a general way by saying that sensation perceives the different qualities of the stimuli, and feeling only the quantity, the positive or negative action of the stimulus (pleasure or pain). In this last and widest sense we may ascribe the feeling of pleasure and pain (in the contact with qualitatively differing atoms) to all atoms, and so explain the elective affinity in chemistry (synthesis of loving atoms, inclination; analysis of hating atoms, disinclination).