2. The Scientist

Aristotle has traditionally been considered as primarily a philosopher. Perhaps this is a mistake. Let us, if only for a fresh view, consider him chiefly as a scientist.

His curious mind is interested, to begin with, in the process and technique of reasoning; and so acutely does he analyze these that his “Organon,” or Instrument—the name given after his death to his logical treatises—became the textbook of logic for two thousand years. He longs to think clearly, though he seldom, in his extant works, succeeds; he spends half his time defining his terms, and then feels that he has solved the problem. Definition itself he defines definitively as the specification of an object or idea by naming the genus or class to which it belongs (“man is ananimal”) and the specific difference that distinguishes it from all other members of that class (“man is a rational animal”). It is characteristic of his methodical way that he arranged in ten “categories” the basic aspects under which anything may be considered: substance, quantity, quality, relation, place, time, position, possession, activity, passivity—a classification that some writers have found an aid in the amplification of their flagging thought.

He accepts the senses as the only source of knowledge. Universals are generalized ideas, not innate but formed from many perceptions of like objects; they are conceptions, not things.¹⁵⁰ He lays down resolutely, as the axiom of all logic, the principle of contradiction: “It is impossible for the same attribute at once to belong and not to belong to the same thing in the same relation.”¹⁵¹ He exposes the fallacies into which sophists fall or lure us. He criticizes his predecessors for having drawn the universe, or their theories of it, out of their heads, instead of devoting themselves to patient observation and experiment.¹⁵² His ideal of deductive reasoning is the syllogism—a trio of propositions of which the third follows necessarily from the others; but he recognizes that a syllogism, to avoid begging the question, must presuppose a wide induction to make its major premise probable. Though in his philosophical treatises he too often loses himself in deductive reasoning, he lauds induction, accumulates in his scientific works a mass of specific observations, and occasionally records his own or others’ experiments.* With all his errors he is the father of scientific method, and the first man known to have organized co-operative scientific research.

He takes up science where Democritus left it, and dares to enter every field. He is weakest in mathematics and physics, and confines himself there to a study of first principles. He seeks in the Physics not new discoveries but clear definitions of the terms used—matter, motion, space, time, continuity, infinite, change, end. Motion and space are continuous, they are not made up, as Zeno assumed, of small indivisible moments or parts; the “infinite” exists potentially, but not actually.¹⁵³ He feels, though he does nothing to solve, the problems that were to arouse Newton—inertia, gravity, motion, velocity; he has some idea of the parallelogram of forces, and states the law of the lever: “The moving weight will more easily move” (the object) “the farther away it is from the fulcrum.”¹⁵⁴

He argues that the heavenly bodies—certainly the earth—are spherical, for only a spherical earth could explain the shape of the moon when it is eclipsed by the intervention of the earth between it and the sun.¹⁵⁵ He has an admirable sense of geological time; periodically but imperceptibly, he tells us, the sea is replaced by land and land by the sea;¹⁵⁶ countless nations and civilizations have appeared and disappeared, whether through swift catastrophe or slow time: “Probably every art and philosophy has been repeatedly developed to the utmost and has perished again.”¹⁵⁷ Heat is the chief agent of geological and meteorological changes. He hazards explanations of clouds, fog, dew, frost, rain, snow, hail, wind, thunder, lightning, the rainbow, and meteors. His theories are often bizarre; but the epochal importance of the little treatise on meteorology is that it invokes no supernatural agencies, but seeks to account for the apparent whims of the weather through natural causes operating in certain sequences and regularities. Natural science could go no further until invention gave it instruments of greater scope and precision in observation and measurement.

It is in biology that Aristotle is most at home, observes most widely and abundantly, and makes the most mistakes. The consolidation of previous discoveries in the final establishment of this vital science is his supreme achievement. With the help of his pupils, he gathered data on the fauna and flora of the Aegean countries, and brought together the first scientific collections of animals and plants. If we may follow Pliny,¹⁵⁸ Alexander gave orders to his hunters, gamekeepers, fishermen, and others to supply Aristotle with whatever species and information he might request. The philosopher apologizes for his interest in lowly things: “In all natural objects there lies some marvel, and if any one despises the contemplation of the lower animals, he must despise himself.”¹⁵⁹

He classifies the animal kingdom into enaima and anaima—blooded and bloodless—approximately corresponding to our “vertebrates” and “invertebrates.” He subdivides the bloodless animals into testaceans, crustaceans, mollusks, and insects; the sanguineous into fishes, amphibians, birds, and mammals. He covers an impressively vast and varied field: organs of digestion, excretion, sensation, locomotion, reproduction, and defense; the types and ways of fishes, birds, reptiles, apes, and hundreds of other groups; their pairing seasons and their methods of bearing and rearing their young; the phenomena of puberty, menstruation, conception, pregnancy, abortion, heredity, twins; the habitats and migrations of animals, their parasites and diseases, their modes of sleep and hibernation. . . . He gives an excellent account of the life of the bee.¹⁶⁰ He is full of queer incidental observations: that the blood of oxen coagulates more rapidly than that of most other animals; that some male animals, especially the goat, have been known to give milk; that “in both sexes the horse is the most salacious of animals after man.*¹⁶¹

He is particularly interested in the reproductive structures and habits of animals, and marvels at the multiplicity of ways in which nature achieves the continuance of species, “preserving the type when she is unable to preserve the individual”;¹⁶² in this field his work remained unequaled until the last century. The life of animals moves about two foci—eating and procreation.¹⁶³ “The female has an organ which must be regarded as an ovary, for it contains that which at first is undifferentiated egg, and which becomes by differentiation many eggs.”†¹⁶⁴ The female element contributes to the embryo material and food, the male element contributes energy and movement; the female is the passive element, the male is the activating agent.¹⁶⁵ Aristotle rejects the opinions of Empedocles and Democritus, that the sex of the embryo is determined by the temperature of the womb, or by the preponderance of one reproductive element over the other, and then reformulates the theories as his own: “Whenever the formative (male) principle fails to gain the upper hand, and from deficient warmth fails properly to cook the material and so fashion it into its own shape, then will this material pass over into . . . the female.”¹⁶⁶ “Sometimes,” he adds, “women bring forth three or even four children, especially in certain parts of the world. The largest number ever brought forth is five, and such an occurrence has been witnessed on several occasions. There was once upon a time a woman who had twenty children at four births; and most of them grew up.”¹⁶⁷

He anticipates many theories of nineteenth-century biology. He believes that the organs and characteristics of the embryo are formed by tiny particles (the “gemmules” of Darwin’s “pangenesis”) that pass from every part of the adult into the reproductive elements.¹⁶⁸ Like Von Baer he teaches that in the embryo the characters belonging to the genus appear first, those belonging to the species second, those belonging to the individual third.¹⁶⁹ He states a principle on which Herbert Spencer prided himself, that the fertility of organisms, by and large, varies inversely as the complexity of their development.¹⁷⁰ His description of the chick embryo shows him at his best:

If you wish, try this experiment. Take twenty or more eggs and let them be incubated by two or more hens. Then each day, from the second to that of hatching, remove an egg, break it, and examine it. . . . With the common hen the embryo becomes first visible after three days. . . . The heart appears like a speck of blood, beating and moving as though endowed with life; and from it two veins with blood in them pass in a convoluted course, and a membrane carrying bloody fibers from the vein-ducts now envelops the yolk. . . . When the egg is ten days old, the chick and all its parts are distinctly visible.¹⁷¹

The human embryo, Aristotle believes, develops like the chick: “In the same way the infant lies within its mother’s womb . . . for the nature of the bird can be likened to that of man.”¹⁷² His theory of analogous organs enables him to see the animal world as one: “A nail is the analogue of a claw, a hand of a crab’s nipper, a feather of a fish’s scale.”¹⁷³ At times he comes close to a doctrine of evolution:

Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation. . . . Thus, next after lifeless things in the upward scale comes the genus of plants, relatively lifeless as compared with animals, but alive as compared with corporeal objects. There is in plants a continuous scale of ascent towards the animal. There are certain objects in the sea concerning which one would be at a loss to determine whether they be animal or vegetable. . . . The sponge is in every respect like a vegetable. . . . Some animals are rooted, and perish if detached. . . . In regard to sensibility, some animals give no sign of it, others indicate it obscurely. . . . And so throughout the animal scale there is a graduated differentiation.¹⁷⁴

He considers the ape an intermediate form between man and other viviparous animals.¹⁷⁵ He rejects Empedocles’ notion of the natural selection of accidental mutations; there is no fortuity in evolution; the lines of development are determined by the inherent urge of each form, species, and genus to develop itself to the fullest realization of its nature. There is design, but it is less a guidance from without than an inner drive or “entelechy”* by which each thing is drawn to its natural fulfillment.

Intermingled with these brilliant suggestions there are (as might be expected from the hindsight of twenty-three centuries) errors so numerous, and some so gross, that we are warranted in suspecting that the zoological works of Aristotle have suffered some admixture of his own notes withthose of his students.¹⁷⁶ The History of Animals is a mine of mistakes. We learn there that mice die if they drink in summer; that elephants suffer from only two diseases—catarrh and flatulence; that all animals but man develop rabies when bitten by a mad dog; that eels are generated spontaneously; that only men have palpitation of the heart; that the yolk of several eggs shaken together collects into the middle; that eggs float in strong brine.¹⁷⁷ Aristotle knows the internal organs of animals better than those of men, for neither he nor Hippocrates seems to have overridden religious taboos and practiced human dissection.¹⁷⁸ He thinks that man has only eight ribs, that women have fewer teeth than men,¹⁷⁹ that the heart lies higher than the lungs, that the heart and not the brain is the seat of sensation,*¹⁸⁰ that the function of the brain is (literally) to cool the blood.¹⁸¹ Finally he (or some ponderous proxy) carries the theory of design to depths that make the judicious smile. “It is evident that plants are created for the sake of animals, and animals for the sake of men.” “Nature has made the buttocks for repose, since quadrupeds can stand without fatigue, but man needs a seat.”¹⁸² And yet even this last passage reveals the scientist: the author takes it for granted that man is an animal, and seeks natural causes for the anatomical differences between beasts and men. All in all, the History of Animals is Aristotle’s supreme work, and the greatest scientific product of fourth-century Greece. Biology waited twenty centuries for its equal.