In his own time he was rather reproached for what was considered an undignified and sordid familiarity with observed facts. His critics said that having squandered his patrimony, he had served in the army, and, failing there, had become a _seller of drugs_. His observations on the effects of heat seem to have been drawn from the common processes of the home and the workshop. Even in the ripening of fruits heat appears to him to have a cooking effect. Heat distorts articles made of potters' clay after they have been hardened by cold. Again we find him describing the manufacture of potash and of steel. He is not disdainful of the study of the lower animals, but invites us to investigate all forms in the expectancy of discovering something natural and beautiful. In a similar spirit of scientific curiosity the Aristotelian work _The Problems_ studies the principle of the lever, the rudder, the wheel and axle, the forceps, the balance, the beam, the wedge, as well as other mechanical principles.
In Aristotle, in fact, we find a mind exceptionally able to form clear ideas, and at the same time to observe the rich variety of nature. He paid homage both to the multiplicity and the uniformity of nature, the wealth of the phenomena and the simplicity of the law explaining the phenomena. Many general and abstract ideas (category, energy, entomology, essence, mean between extremes, metaphysics, meteorology, motive, natural history, principle, syllogism) have through the influence of Aristotle become the common property of educated people the world over.
Plato was a mathematician and an astronomer. Aristotle was first and foremost a biologist. His books treated the history of animals, the parts of animals, the locomotion of animals, the generation of animals, respiration, life and death, length and shortness of life, youth and old age. His psychology is, like that of the present day, a biological psychology. In his contributions to biological science is manifested his characteristic inclination to be at once abstract and concrete. His works display a knowledge of over five hundred living forms. He dissected specimens of fifty different species of animals. One might mention especially his minute knowledge of the sea-urchin, of the murex (source of the famous Tyrian dye), of the chameleon, of the habits of the torpedo, the so-called fishing-frog, and nest-making fishes, as well as of the manner of reproduction of whales and certain species of sharks. One of his chief contributions to anatomy is the description of the heart and of the arrangement of the blood-vessels. A repugnance to the dissection of the human body seems to have checked to some extent his curiosity in reference to the anatomy of man, but he was acquainted with the structure of the internal ear, the passage leading from the pharynx to the middle ear, and the two outer membranes of the brain of man. Aristotle's genius did not permit him to get lost in the mere details of observed phenomena. He recognized resemblances and differences between the various species, classified animals as belonging to two large groups, distinguished whales and dolphins from fishes, recognized the family likeness of the domestic pigeon, the wood pigeon, the rock pigeon, and the turtle dove. He laid down the characteristics of the class of invertebrates to which octopus and sepia belong. Man takes a place in Aristotle's system of nature as a social animal, the highest type of the whole series of living beings, characterized by certain powers of recall, reason, deliberation. Of course it was not to be expected that Aristotle should work out a fully satisfactory classification of all the varieties of plants and animals known to him. Yet his purpose and method mark him as the father of natural science. He had the eye to observe and the mind to grasp the relationships and the import of what he observed. His attempt to classify animals according to the nature of their teeth (dentition) has been criticized as unsuccessful, but this principle of classification is still of use, and may be regarded as typical of his mind, at once careful and comprehensive.