The possibility of a scientific method is based on a rational interpretation of objective phenomena. Psychology, as a science, would be a solemn version of _Alice Through the Looking Glass_ in which real persons are viewed and analysed by their images, or rather by some other intelligence, such as a dog or an inhabitant of Mars. The futility of such a process should be apparent if we recollect that an animal’s mental processes can themselves be estimated and expressed only as a vaguer and more rudimentary sort of human mind. II Our modern idea that science embraces all kinds of knowledge is, as I have mentioned before, a curious reversion to the Greeks. In spite of the fact that they formulated the great deductive laws of physics, such as cause and effect, conservation of matter, etc., and developed an extraordinarily fruitful science of geometry, they remained indifferent to the experimental method. They developed almost no apparatus for experimentation, established no standards of measurement, and their arithmetical symbols were so awkward that only the simplest calculations could be made. They never succeeded in disentangling subjective and objective ideas. Plato could anticipate the modern conception of natural law in his aphorism that God geometrises, but at the same time he vaguely identified the human soul with the stars and endued the universe with life. Democritus pictured the world as an aggregation of atoms, differing only in size, shape, and motion; yet he also tacitly ascribed to them a will or desire to move which was only less pronounced than in those finer particles which constituted the souls of men. Also the four classic elements,--earth, water, air, and fire,--which combined in different proportions to form all material bodies, were actuated by the animistic principle that each element sought its own place. One could multiply these examples of classical thought which confused mechanical forces with the vital attributes of will and desire.