Since we regard the whole of organic life as, in the ultimate analysis, merely a very elaborate chemical process, we shall quite expect that chemical stimuli are the most important factors in sensation. And this is so in point of fact; from the simplest moneron up to the most highly differentiated cell and on to the flower in the plant and the mental life of man, the vital processes are dominated by chemical forces and conversions of energy, which are set in play by external or internal chemical stimuli. The excitation which they produce is called, in a general way, "sensation of matter" or chemæsthesis; the basis of it is the mutual relation of the chemical elements which we describe as chemical affinity. In this affinity we have the play of attractive forces which lie in the nature of the elements themselves, especially in the peculiar properties of their constituent atoms; and this cannot be explained unless we ascribe unconscious sensation (in the widest sense) to the atoms, an inherent feeling of pleasure and the reverse, which they experience in the contact of other atoms (the "loves and hatreds of the elements" of Empedocles). The numbers of different stimuli that act chemically on the plasm and excite its "sensation of matter" may be divided into two groups--external and internal stimuli. The latter lie within the organism itself, and cause the internal "organic sensations"; the former are in the outer world, and are felt as taste, smell, sex-impulse, etc. In the higher animals special chemical sense-organs have been developed for these chemical stimuli. As these are well known to us from our own human experience, and comparative physiology shows us the same structures in the higher animals, we will deal first with them. In general the same law holds for these external chemical stimuli as for optical and thermic stimuli; we can recognize a maximum limit of their action, a minimum below which they fail to stimulate, and an optimum or stage in which their influence is strongest.