Medieval Religion, Science, and Astronomy

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Medieval Religion, Science, and Astronomy

During the European Dark Ages there was no coherent system of scientific or philosophical thought. Throughout Western civilization, theological doctrine and dogma replaced the rational and logical inquiry of the ancient Greek scholars. During the thirteenth and fourteenth centuries, however, the rediscovery of Aristotle's (384-322 b.c.) philosophy, as preserved by Arabic scholars, renewed interest in the development of logic and scientific inquiry. The critical writings of St. Thomas Aquinas (1227-1274), Roger Bacon (c. 1214-1292) and William Ockham (also spelled Occam, c. 1285-c. 1349) regarding Aristotelian ideas ultimately laid the intellectual foundations for the seventeenth century scientific revolution by de-emphasizing the primacy of understanding based upon scriptural revelation or authority.


Although the origins of astronomy and cosmology (the study of the origin, structure, and evolution of the universe) predate the human written record, by the height of ancient Greek civilization the cause of natural phenomena was attributed to the collective whim of a pantheon of gods. Although monotheistic in the same sense as ancient Judaism, out of this pantheism (a theology that includes multiple gods) arose the idea that there was an infinite being, Plato's (c. 428-c. 347 b.c.), "The One," and Aristotle's "Prime Mover." Aristotle's influence over astronomy and cosmology was to extend for nearly two millennia and, as a set of philosophical and scientific explanations of the universe, Aristotle's assertions ultimately became integral to the tightly interwoven fabric of philosophy, science, and theology that came to dominate the late medieval intellectual landscape.

In his work De caelo, Aristotle discussed the motion of bodies and the structure of matter, and set forth a model of a finite geocentric universe (an Earth-centered spherical universe). Based on his observations that celestial bodies seemed to move in circular paths and recurring patterns across the night sky, Aristotle argued that the cosmos was not infinite. In contrast to a Platonic distaste for actual observation, however, Aristotle also argued for a spherical Earth based upon his observation of the shadows of the Earth on the Moon and of the progressive disappearance of ships over the horizon. The simplicity and reproducibility of Aristotle's arguments allowed a degree of secularization in the study of natural phenomena. This secularization allowed Aristotle to advance the concept of an internal or external "mover" as something that puts a body into motion from a state of absolute rest. Accordingly, once put in motion, bodies required the continued intervention of a "mover" to continue their motion. Intuitively, Aristotle argued that there a must be a "prime mover" of all things who continued to direct movement, especially the observed motion of celestial bodies.

Aristotle proposed that the cosmos was composed of concentric, crystalline spheres to which the celestial objects were attached and upon which they moved. The outermost sphere was the domain of the prime mover who caused the outermost sphere to rotate at constant angular velocity. This motion of the outer sphere imparted movement to inner spheres that resulted in the observable movements of planets and stars. Based on Aristotelian concepts, in a.d. 150 the Greek astronomer Ptolemy developed a model of an Earth-centered cosmos composed of concentric crystalline spheres that was to dominate the Western intellectual tradition until challenged by the Polish astronomer Nicolaus Copernicus (1473-1543). Ptolemy's model, as published in his Almagest, featured epicycles (movement of the planets about smaller circles imposed on greater circles) that allowed practical adjustments of planetary motion to account for retrograde motion (the apparent temporary reversal of the motion of planets on the celestial sphere later explained by planets overtaking one another in their respective orbits) and varying planetary brightness.

Aristotle's logic and Ptolemy's model presented a well-developed—seemingly intellectually invulnerable—rational explanation of the cosmos that left little room for faith. The fall of the Roman Empire, however, plunged Western civilization into the Dark Ages during which most of Greek philosophy and reasoning was lost in Europe.


The Western rediscovery of Aristotle's philosophy in the twelfth century brought the theologically dominated medieval world into sharp conflict with Aristotelian logic as contained in the Book of Causes and other works of Arabic scholars. Although the original author of the Book of Causes is unknown (some scholars argue that it originated in ninth or tenth century Baghdad, others that it was written in twelfth century Spain), the book elaborated upon Aristotelian concepts by asserting that from each cause there results a certain order in its effects. In contrast to medieval beliefs regarding miracles, the Book of Causes argued that God could not do anything contrary to the order he had already established. Aristotle's concept of a prime mover as a being that itself remains unmoved, unchanging, and impersonal was also incompatible with the Christian concept of a God who regularly intervened in the affairs of humans through miracles. In addition, Aristotle's argument that the universe was circular and eternal contrasted with the Christian doctrine of creation.

Most early medieval scholars rejected the eternity of the universe as philosophically absurd. Some however, made tenuous connections to Aristotle's prime mover by asserting that God was ultimately the cause of all phenomena and that God worked through natural mechanisms.

As part of a broader effort to harmonize logic with religious revelation (i.e., Greek logic and rationality with Christian monotheism), Thomas Aquinas attempted to harmonize the work of Aristotle with medieval theological doctrine. Aquinas stated his own purposes in his Commentary on the Book of Causes as an attempt to "delineate the first causes of things" because the "ultimate happiness possible in this life must lie in the consideration of first causes, because what little we can know about them is worthier of devotion and nobler than all that we can know about lower things."

For Aquinas, reason and revelation were both legitimate paths to knowledge and truth and therefore both philosophic and scientific inquiry could be used to prove theological concepts. In his work Summa Theologica, Aquinas set forth a comprehensive defense of Christian doctrine. Aquinas claimed five proofs for the existence of God based on motion, efficient causes, necessity and contingency, goodness, and teleology in nature.

Aquinas's first "proof" of the existence of God was, in fact, based on the Aristotelian concept of a prime mover. Aquinas argued that the stars needed a continuing influence or source of motion and that they must therefore have had a "prime" or "first mover" who Aquinas identified as the Christian God. In accordance with the assignment of the role of prime mover in Aristotelian cosmology to the Christian God, the outermost celestial sphere became identified with the Christian heaven.

The classical notion of causality is succinctly expressed in Aquinas's Commentary on Aristotle's Physics in which Aquinas achieved a synthesis of Greek pagan religion with Christian theology and, in so doing, challenged the traditional reasoning that asserted that understanding came only from belief. With the synthesis of Aquinas, this philosophical process was reversed so that belief came from, or was strengthened by, a deeper understanding of nature.

For Aquinas there was an absolute and direct correlation between natural phenomena and divine revelation. The writings of Aquinas can be viewed within the larger context of an effort to characterize God as both the creator and continuing guiding force behind physical phenomena. Accordingly, descriptions of nature could be fully explained by logical inquiry and the human mind—as created by God—could be relied upon to produce more reliable descriptions of nature. Another important contribution to the renewed interest in scientific thought resulted from Roger Bacon's work in optics, upon which Bacon based assertions of the usefulness of natural knowledge as means to support religious belief.

Aquinas's synthesis was not, however, to go unchallenged. Early in the fourteenth century William Ockham and others began concerted studies into the causes of motion. Ockham soon rebelled against Aristotelian physics by advancing the impetus theory of movement. The impetus theory rejected the need for a mover to be in contact with a body in order for a body to continue its motion. According to Ockham, bodies continued their motion due to their own impetus, a forerunner of modern concepts regarding momentum.

Ockham cautioned against too great a reliance on logical inquiry. For Ockham, the movements of bodies and celestial objects could be described in terms of the properties of matter. More importantly, the work of Ockham showed the fallibility of reliance on intuitively obvious or logical explanations. Moreover, in contrast to the Aristotelian assertion that thoughts and ideas were a type of real and objective matter, Ockham asserted that reality was based upon tangibility in space and time, not upon metaphysical logic.

In his arguments, Ockham coined a maxim which has since become known as Ockham's razor (entia non sunt multiplicanda praeter necessitatem) that asserts that the best scientific theory, all other factors being equal, is the explanation or theory which requires the fewest new starting assumptions. This principle of parsimony (simple explanation) was used by Ockham to refute the need for the numerous entity-laden and complex models that had been devised to explain reality. Ockham's razor is best expressed in his assertion, "Nulla pluralitas est ponenda nisi per rationem vel experientiam vel auctoritatem illius, qui non potest falli nec errare, potest convinci" (No plurality should be assumed unless it can be proved by reason, experience, or by infallible authority) and remains a well-accepted means of choosing between two theories regarding the same physical phenomena.

Many centuries away from the empiricism that was to fuel the scientific revolution, Ockham and other nominalist philosophers ultimately fell back upon the maxim that faith was a more reliable guide than reason. Regardless, the writings of Aquinas, Bacon, and Ockham greatly contributed to a reestablishment of reason and logic as a path of knowledge and ultimately deemphasized the role of understanding based upon mystical scriptural revelation.


Further Reading

Aquinas, St. Thomas. Commentary on Aristotle's Physics. Translated by Blackwell, R., J. Spath, and W. Thirlkel. South Bend, IN.: Dumb Ox Bks., 1998.

Aquinas, St. Thomas. Summa Theologica. Allen, TX.: Christian Classics, Inc., 1998.

Burtt, Edwin A. The Metaphysical Foundations of Modern Physical Science. Atlantic Highlands, N.J.: Humanities Pr. International, Inc., 1982.

Gilson, Etienne. The Spirit of Medieval Philosophy. Notre Dame, IN.: Univ. of Notre Dame Pr., 1991.

Hetherington, Norriss S. Enclyclopedia of Cosmology: Historical, Philosophical, and Scientific Foundations of Modern Cosmology. New York: Garland Publishing, Inc., 1993.

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Medieval Religion, Science, and Astronomy

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