Aristotle pressed his way through the mass of things knowable, and subjected its diversity to the power of his thought. No wonder that for ages he was known as "The Philosopher," master of those who know. His purpose was to comprehend, to define, to classify the phenomena of organic and inorganic nature, to systematize the knowledge of his own time. Twenty years' apprenticeship in the school of Plato had sharpened his logical powers and added to his stock of general ideas, but had not taught him to distrust his senses. When we say that our eyes deceive us, we really confess that we have misinterpreted the data that our sight has furnished. Properly to know involves the right use of the senses as well as the right use of reason. The advance of science depends on the development both of speculation and observation. Aristotle advised investigators to make sure of the facts before seeking the explanation of the facts. Where preconceived theory was at variance with observed facts, the former must of course give way. Though it has been said that while Plato was a dreamer, Aristotle was a thinker, yet it must be acknowledged in qualification that Plato often showed genuine knowledge of natural phenomena in anatomy and other departments of study, and that Aristotle was carried away at times by his own presuppositions, or failed to bring his theories to the test of observation. The Stagirite held that the velocity of falling bodies is proportional to their weight, that the function of the diaphragm is to divide the region of the nobler from that of the animal passions, and that the brain is intended to act in opposition to the heart, the brain being formed of earthy and watery material, which brings about a cooling effect. The theory of the four elements--the hot, the cold, the moist, the dry--led to dogmatic statements with little attempt at verification. From the standpoint of modern studies it is easy to point out the mistakes of Aristotle even. Science is progressive, not infallible.