(1903) [also in Dr. Stout's recent little manual of psychology and Mr. W. H. Mallock's _Religion as a Credible Doctrine_]. XIII SENSATION Sensation and consciousness--Unconscious and conscious sensation--Sensibility and irritability--Reflex sensation and perception of stimuli--Sensation and living force--Reaction to stimuli--Resolution of stimuli--External and internal stimuli--Conveyance of stimuli--Sensation and striving--Sensation and feeling--Inorganic and organic sensation--Light sensation, phototaxis, sight--Sensation of warmth, thermotaxis--Sensation of matter, chemotaxis--Taste and smell--Erotic chemicotropism--Organic sensations--Sensation of pressure--Geotaxis--Sensation of sound--Electric sensation. Sensation is one of those general terms that have at all times been liable to the most varied interpretations. Like the cognate idea of the "soul," it is still extremely ambiguous. During the eighteenth century it was generally believed that the function of sensation was peculiar to animals, and was not present in plants. This opinion found its most important expression in the well-known principle in Linné's _Systema Naturæ_: "Stones grow: plants grow and live: animals grow, live, and feel." Albrecht Haller, who gathered up all the knowledge of his time relating to organic life in his _Elementa Physiologiæ_ (1766), distinguished as its two chief characters "sensibility" and "irritability." The one he ascribed exclusively to the nerves, and the other to the muscles. This erroneous idea was subsequently refuted, and in our own time irritability is conceived to be a general property of all living matter. The great advance made by the comparative anatomy and experimental physiology of animals and plants in the first half of the nineteenth century brought to light the fact that irritability or sensibility is a common quality of all organisms, and that it is one of the principal characteristics of vital force (_cf._ chapter ii.). The greatest merit in connection with its experimental study attaches to the famous Johannes Müller. In his classical _Manual of Human Physiology_ (1840) he established his theory of the specific energy of the nerves and their dependence on the sense-organs on the one hand and the mental life on the other. He devoted the fifth chapter of his book to the former and the sixth to the latter, approaching particularly to Spinoza in his general psychological views; he treated psychology as a part of physiology, and thus put on a sound scientific basis that naturalistic conception of the place of psychology in the biological system which we now regard as the correct view. At the same time he proved that sensation is a function of the organism as much as movement or nutrition.