In the investigation of purely physical processes we generally employ notions so abstract that as a rule we only think cursorily or not at all of the sensations that lie at and constitute their foundation. For example, when I establish the fact that an electric current of 1 Ampère develops 10½ cubic centimetres oxyhydrogen gas at 0° C. and 760 mm mercury pressure in a minute, I am easily disposed to attribute to the objects defined a reality wholly independent of my sensations. But I am obliged in order to arrive at what I have determined to conduct the current through a circular wire having a definite measured radius, so that the current, the intensity of terrestrial magnetism being given, shall turn the magnetic needle at its centre a certain angular distance out of the meridian. The intensity of terrestrial magnetism must have been disclosed by a definite observed period of vibration of a magnetic needle of measured dimensions, known weight, and so forth. The determination of the oxyhydrogen gas is no less intricate. The whole statement, so simple in its appearance, is based upon an almost unending series of simple sensory observations (sensations), particularly so when the observations are added that guarantee the adjustment of the apparatus, which may have been performed in part long before the actual experiment. Now it may easily happen to the physicist who does not study the psychology of his operations, that he does not (to reverse a well-known saying) see the trees for the woods, and that he slurs over the sensory elements at the foundation of his work. Now I maintain, that every physical notion is nothing more than a definite connection of the sensory _elements_ which I denote by _A_ _B_ _C_ ..., and that every physical fact rests therefore on such a connection. These _elements_—elements in the sense that no further resolution has for the present been effected of them—are the most ultimate building stones of the physical world that we have as yet been able to seize. Physiological research also may have a purely physical character. I can follow the course of a physical process as it propagates itself through a sensitive nerve to the spinal column and brain of an animal and returns by various paths to the muscles of the animal, whose contraction produces further events in the environment of the animal. I need not think, in so doing, of any feeling on the part of the animal; what I investigate is a purely physical object. Very much is lacking, it is true, to our complete comprehension of the details of this process, and the assurance that it is all motion can neither console me nor deceive me with respect to my ignorance.