On the Motion and Gait of Animals

Summary

This record joins two brief Aristotelian treatises that ask how a living thing gets from one place to another. The Motion of Animals (De Motu Animalium, eleven short chapters) asks what all animal movement has in common; the Gait of Animals (De Incessu Animalium, nineteen chapters, often called the Progression of Animals) asks why particular animals have the limbs they have and bend them as they do. Read together they move from cosmology to the knee joint and back again. The Motion of Animals opens with a mechanical principle: nothing that moves itself can do so unless something is at rest. A bent elbow pivots on a point that stays still while the forearm swings; a walker needs ground that does not give way, as mice slip in heaped grain and men in sand; a man inside a boat cannot pole it forward by pushing against the boat itself. Aristotle then widens the principle to the heavens, criticising the myth of Atlas and those who make the poles a source of motion, and quoting Homer's Zeus, whom no god can drag down by a golden chain, to argue that the first mover of the universe must lie outside what it moves. The centre of the treatise is psychological. Chapter 6 names the movers of an animal as intellect, imagination, purpose, wish and appetite, and reduces them to thought and desire; the object desired, a real or apparent good, moves without being moved, as the prime mover does. Chapter 7 then gives Aristotle's best-known account of what later readers call the practical syllogism: two premises, one about the good and one about the possible, issue not in a proposition but in an action. "I need a covering, a coat is a covering": the man makes a coat. Aristotle compares the body to automatic puppets and a child's toy cart, but notes that living parts also swell and shrink with heat and cold, so that a mere image can make us shudder or blush. Chapters 9-11 locate the soul's moving origin in the middle of the body, near the heart, name the connate spirit (pneuma) as its instrument, liken the well-ordered body to a city that needs no magistrate at every post, and separate voluntary movement from involuntary movements of the heart and genitals and non-voluntary ones such as sleep and breathing. The Gait of Animals turns comparative. Starting from the rule that nature does nothing in vain, Aristotle defines the body's six directions, explains why blooded animals move at no more than four points, why snakes have no legs, why man alone stands upright, why birds have two legs and wings rather than four legs, and why men and quadrupeds bend their limbs in opposite directions. The treatise is an early attempt to explain anatomy by function.

Top Lessons

• Every push needs a place to stand

  Aristotle's first claim is physical and easily tested: a joint moves only because one point in it is still, and an animal advances only by pressing on something that resists. The sand, the heaped grain and the boat give students experiments they can perform. The principle then grounds his argument that the universe itself depends on a mover outside it.

• The coat syllogism: how a reason becomes a deed

  In chapter 7 Aristotle says that practical reasoning concludes not in a statement but in an action: I need a covering, a coat is a covering, so I make a coat. He adds that we often skip the obvious premise, and that appetite can supply the conclusion at once (I want to drink; this is drink). This passage is a starting point for his ethics of deliberation and for many later accounts of moral action.

• An image can move the body

  Aristotle notes that imagined objects affect the body as real ones do: we shudder at a mere idea, blush and turn pale, and memories and anticipations bring the same changes of heat. A tiny change at the centre, like a rudder moved a hair's breadth, makes a large change at the periphery. The passage invites careful attention to what we allow ourselves to imagine and dwell on, a theme Christian moral teachers also took up when they spoke of guarding the heart and the thoughts.

• The body as a well-governed city

  At the end of chapter 10 Aristotle compares the organism to a commonwealth where, once order is established, no separate ruler is needed at each task; the soul resides centrally and the parts do their work by natural structure. The image describes a hierarchy in which order is distributed rather than micromanaged. Students can test it against what he says about involuntary movements, which happen without the ruler's command.

• Nature makes nothing in vain: reading the limbs

  The Gait of Animals assumes that each anatomical feature is the best possible for the animal's kind and then asks why: why snakes are limbless, why man alone stands erect with legs heavier than his upper body, why children cannot walk upright at first, why human limbs bend in opposite directions. Some of his answers are wrong, but the method of asking what a structure is for proved durable in later anatomy.

Verified Quotes

Medical Lens

The Motion of Animals is one of the earliest sustained attempts to say how a mental event becomes a bodily movement. Aristotle's chain runs from perception or imagination, through desire, to a change of heat at the heart, carried by the connate spirit (pneuma) to the limbs. He uses this to explain why imagined or remembered objects produce shuddering, blushing, pallor and goose-flesh, and why the heart and the genitals move at an image without the command of reason. He also distinguishes non-voluntary bodily processes such as sleep, waking and respiration. The cardiocentric physiology and the doctrine of connate pneuma were later disputed by Galen and the Alexandrian anatomists, who placed the ruling part in the brain and nerves. Readers should treat these passages as the history of physiological psychology, not as clinical description; any link to modern psychosomatic medicine is an interpretive comparison.

• Motion of Animals, ch. 7

  Images and conceptions alter bodily heat as real objects would; a slight change in the region of the heart produces blushing, pallor, shivering and goose-flesh at the periphery.

• Motion of Animals, ch. 8

  Pleasure and pain, courage, panic and erotic feeling are each accompanied by a change of temperature; memories and anticipations cause the same changes in weaker form.

• Motion of Animals, ch. 10

  Connate spirit, seated in or near the heart, expands and contracts to push and pull, serving as the material instrument of the soul's movement.

• Motion of Animals, ch. 11

  Involuntary movements of the heart and generative organ at an image, and non-voluntary processes such as sleep, waking and breathing, explained by natural changes of temperature.