Abstract

This dissertation argues that Newton's First Law of Motion, also known as the principle of inertia, is compatible with the Aristotelian definition of motion accepted by St. Thomas Aquinas and with the mover causality principle, which follows from that definition. The argument distinguishes Aquinas' general principles of natural philosophy from his more specific principles of nature. Thus, one can abandon Aquinas' geocentric cosmology without rejecting his entire philosophy of nature, which includes principles such as the four causes, the mover causality principle, and the definitions of motion and time. That is, the Aristotelian conception of what motion is as such does not depend upon Aquinas' accounts of gravitational, projectile, and celestial motion.

After explaining what Aquinas means by the definition of motion, "the act of a being in potency insofar as it is in potency," I argue that Aquinas' view of the mover causality principle, "whatever is moved is moved by another," has been frequently misunderstood. Aquinas did not hold that the mover causality principle requires a continuously acting mover to sustain a body's motion. I then show, using Newton's examples of the First Law of Motion, his conception of inertia, the common practice of physicists and astronomers, and other arguments, that the principle of inertia requires a version of the mover causality principle that is compatible with Aquinas' own understanding.

I then argue that the principle of inertia is distinct from the philosophical framework within which it has been commonly and historically conceived. In this framework, motion is no longer thought of as a coming to be but is considered a first principle or an unchanging primary quality, a view that the principle of inertia seemingly instantiates through the notion of a "state." This philosophical framework distorts the understanding of the principle of inertia. I argue that the principle of inertia is more intelligibly understood in light of the Aristotelian definition of motion and from within the framework of Thomistic natural philosophy. The argument considers relevant principles of Special and General Relativity as well as principles, such as energy and fields, which were unknown to Newton but later became part of what we now call Newtonian physics.