Biology was of later development than physics, because it is easier to subject inert matter to experiment and measurement than the living body. But mental phenomena are more multiform, complex, transient, and obscure than those of the living body. It is natural, therefore, that psychology should be the last of the sciences.
5. Mental phenomena are highly complex, but the phenomena of the physical world are not so simple as is commonly assumed.
There are certain physical laws, those concerned with the attraction of gravitation, the dispersion of light, etc., which depend on the geometrical nature of space; others, such as the indestructibility of matter and the correlation of energy, which depend largely on our sense of the fitness of things. Neither of these sorts of laws can be exactly proved by experiment, while it is likely that the first sort will be found to be inexact. We can, however, best consider the boundless complexity of nature by taking laws depending on measurements. Thus the experiments made by Boyle and by Mariotte discovered an apparently simple relation between the volume of a gas and its external pressure. More careful experiments showed that the law held only for certain gases and pressures. Still more exact measurements show that the law is only approximate, the relations between the volume of a gas and its external pressure being as numerous as there are gases, pressures, and temperatures. The movements of the planets may serve for a second example. By Copernicus their orbits were supposed to be circles and their motion uniform. Kepler discovered the less simple relations, which still serve as a convenient generalization. But it is evident that the relative motions of the planets are complex beyond the limits of measurement or mathematical expression. Students of physics are not discouraged by the difficulties involved in the undulatory theory of light, and students of biology continue to work over the intricate problems of heredity. Students of mathematics do not give up their
