very frequently in the experimental sciences that we possess facts without being able to connect them by theories.
According to Kant's law, all our sensations are intensive quantities; that is, they are matters of degree. Can we, then, apply precise mathematical measures to their intensity? Every sensation does, in fact, present itself to us as being more or less strong, and consequently as a magnitude. Then why can we not measure it, like any other magnitude or any quantity? But we must mark a difference between psychological or physiological measures and the physical measures of physicists. As physics measures sounds, light, and heat, it might appear that we should already have been able to measure sensations. But it is obvious that physics measures these qualities only as objective properties of bodies, while the psychological measure of sensations is a quite other question. The present question, for example, is whether two quantities of light, physically and objectively equal, produce equal sensations, and unequal luminous causes produce unequal sensations—or whether, in short, the proportion existing between the causes also exists between the effects. "There is no one," says M. Ribot, "who has not compared two sensations and remarked that one is stronger and the other weaker. We declare without hesitation that there is more light at noonday than in moonlight, and that a cannon-shot makes more noise than a pistol." So far consciousness is sufficient; but this is not what we call measurement from the mathematical point of view. To measure a magnitude mathematically is to find how many times it is contained in another magnitude taken as unity. Has the sun a hundred or a thousand times more light than the moon? Does the cannon make a hundred or a thousand times more noise than the pistol? Such questions can not be answered by the consciousness, which can not tell us how many times one sensation is contained in another. It would naturally occur to the mind that sensation increases in proportion to the excitation, as when Herbart thought that two lights would give twice as much illumination as one. But this is not true. We hear distinctly sounds in the night, or in solitude, which are imperceptible in the daytime or in the hurly-burly of business. A double volume of sound is not produced when the number of instruments or of singers at a concert is doubled. A question is involved, calling for careful discussion in determining the proportion in which sensation is augmented or diminished with the excitation. This is one of the objects of what is called psycho-physics.
The question of heredity is another of the new matters which physiological psychology has introduced into philosophy. Till recently, the factor of heredity has been omitted in psychological treatises. In the schools of Condillac, Reid, and Jouffroy, the in-
