On the Progression of Animals

Aristotle. De animalium motione et De animalium incessu. Jaeger, Werner, editor. Leipzig: Teubner, 1913.

Open Greek and Latin · CC BY-SA 4.0

Cloned and adapted by Humanitext, with ongoing edits.

This work is a scientific and philosophical treatise that investigates the functions, numbers, and structures of animal parts involved in locomotion based on teleological and mechanical principles. Aristotle begins by establishing the premise that "nature does nothing in vain," defining the six spatial dimensions—up/down, front/back, and right/left—according to biological functions and identifying the origin of motion. He then geometrically and mechanically demonstrates that flexion of joints and resistance against a supporting surface are essential for all forms of locomotion, such as walking, flying, and swimming, asserting that animals must possess an even number of limbs. The treatise systematically analyzes various creatures, including humans, quadrupeds, birds, fish, insects, and multipeds, explaining how the number of their limbs and the direction of their joint flexions are rationally adapted to their environments and mechanical efficiency. Finally, after examining the peculiar movements of creatures like crabs and flatfish, the author concludes his discussion of animal bodies and motion, transitioning toward the study of the soul.