Jean Buridan

This French priest (1295-1358) sowed the seeds of the Copernican revolution in Europe. His most famous work is the Summulae de dialectica (Compendium of Dialectic), a text of amazing breadth and originality aimed at redeeming the older tradition of Aristotelian logic by using the newer logical forms such as those in use by Peter of Spain (see Pope John XXI).  But his importance to science lay primarily in taking the first step toward the modern understanding of mechanics, an important development in the history of medieval science. Through study of a spinning top, he developed the idea of impetus, mass, velocity and resistance, all critical to creating the concept of inertia. Galileo and Newton depended on his insights.

The concept of inertia was alien to the physics of Aristotle. Aristotle, and his followers held that a body was only maintained in motion by the action of a continuous external force. Thus, in the Aristotelian view, a projectile moving through the air continues to move due to eddies or vibrations in the surrounding air. Planets could only move if someone or something were pushing them. According to Aristotle, without an external push, even in the absence of an opposing force, a moving body would come to rest almost immediately.

Jean Buridan, however, noticed that common experience proved Aristotle wrong. By studying the movement of a child’s toy, a spinning top, he followed in the footsteps of John Philoponus and Avicenna, and proposed that motion was maintained by some property of the body, imparted when it was set in motion. Buridan named the motion-maintaining property “impetus.” Moreover, he rejected the view that the impetus dissipated spontaneously (this is the big difference between Buridan’s theory of impetus and his predecessors). Instead, he asserted that a body would be arrested by the forces of air resistance and gravity which might be opposing its impetus. Buridan further held that the impetus of a body increased with the speed with which it was set in motion, and with its quantity of matter. Clearly, Buridan’s impetus is closely related to the modern concept of momentum. Buridan saw impetus as causing the motion of the object. He anticipated Isaac Newton when he wrote:

...after leaving the arm of the thrower, the projectile would be moved by an impetus given to it by the thrower and would continue to be moved as long as the impetus remained stronger than the resistance, and would be of infinite duration were it not diminished and corrupted by a contrary force resisting it or by something inclining it to a contrary motion (Questions on Aristotle’s Metaphysics XII.9).

In Book VIII, Question 12 of his work Super octo libros physicorum Aristotelis subtilissimae quaestiones, Buridan turned this reasoning toward the heavens and noted that the Bible does not claim that God had to keep his hand on the celestial bodies to maintain their motion. Buridan instead pointed out the motion of celestial bodies could be solved a different way. “God, when He created the world, moved each of the celestial bodies as He pleased, and in moving them He impressed in them impetuses which moved them without His having to move them any more except by the method of general influence whereby He concurs as a co-agent in all things which take place.” With these words, Buridan introduced the concepts that would lead to Newton’s first law of motion: a body at rest would stay at rest and a body in motion would stay in motion with the same speed and in the same direction unless acted upon by another force. Since Buridan proposed that the motion of heavenly bodies was governed by the same laws that applied to physical bodies on earth, he also anticipated the universality of Newton’s laws of motion.