While skepticism and doubt have had a presence in human thought for nearly as long as religious faith has existed, they have had a place within religious thought rather than in opposition to it for the vast majority of their existence. Doubt was generally employed by religious thinkers for the purpose of strengthening and explaining their faith, as can be seen in the numerous “proofs” for the existence of God formulated by the great theologians of the Middle Ages, such as Thomas Aquinas and Anselm of Canterbury. With the new science and philosophy of the Enlightenment, however, unbelief began to be seen as a viable alternative option that stood in opposition to faith. In addition to the popular deism of the Enlightenment, espoused by such important figures as Voltaire and Maximilien Robespierre, atheism also found its first explicit adherents among such figures of the French Enlightenment as Baron d'Holbach and Jacques André Naigeon. This new view of disbelief would have a major influence on subsequent generations of thinkers in the West as proponents of religion now had to contend with disbelief as a rival system of thought and many of the most influential philosophies, such as those of Friedrich Nietzsche and Karl Marx, supported and often assumed this concept of disbelief. Among the numerous new concepts introduced by the philosophers of the Enlightenment, one of those which have had the longest lifespan and the greatest impact has been the introduction of disbelief as a viable alternative position to religious faith, Atheism. One of the most central philosophical pursuits of the Middle Ages was the attempt to reconcile faith and reason. Medieval thinkers had inherited both the religious tradition of the ancient Middle East, which they saw as representative of faith, and the philosophical tradition of ancient Greece, which they saw as representative of reason. In their attempts to synthesize the two, the primary question they encountered was whether the existence of God, the primary object of faith, could be proved through the use of reason alone. “Some of the greatest thinkers who have ever lived have pored at length over this question.”
One of the most remarkable features of Medieval philosophy is the centrality of this question when compared with the apparent nonexistence of any separate class of nonbelievers. Not only are there no surviving writings by or about any person espousing outright unbelief during the Middle Ages, but according to Sarah Stroumsa, “in the discussions of God's existence the actual opponents” of the philosophers examining the question “are not identified as individuals. As a group they are sometimes referred to as heretics, unbelievers, materialists, or skeptics.”
Some of the greatest minds of the Middle Ages, then, dedicated large portions of their work to arguing against an entirely theoretical unbelief. When Anselm of Canterbury formulated his ontological argument and Thomas Aquinas formulated his famous “five ways” to prove the existence of God, they themselves assumed doubt in their writings in order to strengthen faith through reason and to demonstrate that faith and reason are compatible and complimentary.
Later, in the fifteenth century, however, William of Occam set about undoing the synthesis which had been accomplished by Anselm, Aquinas, and others like them. Occam believed that “logic and theory of knowledge had become dependent on metaphysics and theology” as a result of their work and that they had made reason subservient to faith. He “set to work to separate them again.” As a result of his work to separate faith and reason, according to Richard Tarnas, there arose the psychological necessity of a double-truth universe. Reason and faith came to be seen as pertaining to different realms, with Christian philosophers and scientists, and the larger educated Christian public, perceiving no genuine integration...