The medieval principle of motion and the modern principle of inertia

Edward Feser

Proceedings of the Society for Medieval Logic and Metaphysics: Volume 10, (2012)

Abstract

Aquinas’s First Way of arguing for the existence of God famously rests on the Aristotelian premise that “whatever is in motion is moved by another.” Let us call this the “principle of motion.” Newton’s First Law states that “every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.” Call this the “principle of inertia.” It is widely thought that the principle of motion is in conflict with the principle of inertia, and that modern physics has therefore put paid to medieval theology, or at least to its notion of God as the Unmoved Mover of the world.

The assumption is that Aquinas and other Scholastics held that an object cannot keep moving unless something is continuously moving it, but that Newton showed that it is simply a law of physics that once set in motion an object will remain in motion without any such mover. Hence Anthony Kenny judges that “it seems that Newton’s law wrecks the argument of the First Way.” Common though this view is, it is not only mistaken, but unfounded. To think otherwise requires reading into each of the principles in question claims they do not make.


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