By "organic sensations" modern physiology understands the perception of certain internal bodily states, which are mostly brought about by chemical stimuli (to a small extent by mechanical and other irritation) in the organs themselves. As subjective feelings of the organism itself these states are most aptly called "feelings"--the positive states, pleasure, comfort, delight; the negative, discomfort, pain, etc. These organic sensations (also called common sensations or feelings) are of great importance for the self-regulation of the complicated organism. To the positive organic sensations belong not only the bodily feeling of satiety, repose, or comfort, but also the psychic feelings of joy, good humor, mental rest, etc. Among negative common feelings we have not only hunger and thirst, bodily fatigue, bodily pain, sea-sickness, etc., but also mental strain, vertigo, bad humor, and so on. Between the two groups we have the third category of neutral organic sensations, which involve neither pleasure nor pain, but merely the perception of certain internal conditions, such as muscular strain (in lifting heavy objects), the disposal of the limbs (in crossing the legs), and so on. Chemical sensation is just as general and important in organic nature as in the life of organisms. In this case it is nothing less than the basis of chemical affinity. No chemical process can be thoroughly understood unless we attribute a mutual sensation to the atoms, and explain their combination as due to a feeling of pleasure and their separation to a feeling of displeasure. The great Empedocles (fifth century B.C.) explained the origin of all things long ago by the various combination of pure elements, the interaction of love (attraction) and hate (repulsion). This attraction or repulsion is, of course, unconscious, just as in the instincts of plants and animals. If one prefers to avoid the term "sensation," it may be called "feeling" (_æsthesis_), while the (involuntary) movement it provokes may be called "inclination" (_tropesis_), and the capacity for the latter "tropism" (more recently _taxis_, _cf._ chapter xii. of the _Riddle_). We may illustrate it from the simplest case of chemical combination. When we rub together sulphur and mercury, two totally different elements, the atoms of the finely divided matter combine and form a third and different chemical body, cinnabar. How would this simple synthesis be possible unless the two elements _feel_ each other, move towards each other, and _then_ unite?