The Enlightenment was a movement of thought and belief, adopted from interrelated conceptions of God, reason, and nature, to which there was wide assent in Europe during the 17^th and 18^th centuries. Its dominant conviction was that right reasoning could find true knowledge and could lead mankind to felicity. The ancestral ideas of the Enlightenment reach deep into ancient societies. There the first philosophers discovered a regularity in the processes of nature and concluded that the ordering principle was intelligent mind. From the 6^th century B. C. Until Stoicism set the tone for the Roman Empire this assumption of a rational and moral order was an enduring guide to thought. With the fall of the Greek city-states, however, speculation had tended to shift to problems of personal salvation, and the eventual result of this turn to religious values was the victory of Christianity.

    Christianity began early to develop a philosophical content. Its intellectual tools were those inherited from the wreckage of Greek life. With them Christian thinkers hammered out the rigorous system of scholasticism, which Thomas Aquina's brought to a high development in the 13^th century. In Thomas' presentation Aristotle, recently rediscovered by the west, provided the method for obtaining that truth which was ascertainable by reason alone; since Christian revelation contained subordinate to, but not in conflict with, eternal law and divine law.

    Imposing as the Thomistic structure was, it did not perpetuate the church's intellectual dominion. First there were deep rents torn in scholasticism by refined disputation and a sapping of the church's prestige through political controversies. Then came the Renaissance with its worldly outlook and after it the Reformation. Both were less movements for intellectual liberty than changes of authority, but, since they appealed to different authorities, they contributed to the breakdown of the community of thought.

    The great Dutch jurist Hugo Grotius made a pioneering attempt to fashion a new common denominator. In his De jure belli ac pacis (1625) he invoked natural law as the principle governing international relations. For 2000 years no one denied the existence of natural law, but there was a fresh significance in Grotius' emphasis on its primacy and on its independence of Christian theology. This law, said Grotius, was a dictate of right reason and its basic propositions wee as self-evident as 2 x 2 =4.

    It was not by caprice that Grotius appealed to the certainties of an equation. By his time mathematics was developing rapidly and was being used on problems for which Aristotle's answers were no longer satisfactory. The formative influence for the Enlightenment was not so much content as method. The great geniuses of the 17^th century confirmed and amplified the concept of a world of calculable regularity, but, more important, the seemingly proved that rigorous mathematical reasoning offered the means, independent of God's revelation of establishing truth. Hence men as different as Descartes and Hobbes, Spinoza and Leibniz wrote in the idiom of mathematical demonstration.

    If there lingered more faith than certainty, Isaac Newton resolved the issue. Few could understand the mathematics by which in 1686 he stated the law of universal gravitation, but that difficulty did not becloud the great conclusion: Newton had explained the material universe. Newtonianism, then, gave a triumphant vindication to a way of thinking and a fresh incentive to re-examine all problems of human concern.

    The most sensitive of those problems had always been religion. The Christian faith, shaken by a growing secularisation and by embittered dissension, had by the 17^th century to face reports of peoples in the newly discovered countries beyond the seas who, like the revered men of antiquity, lived in virtue an piety although without the light of Christian knowledge. Men at odds with orthodox faith and yet unwilling to turn atheist assumed that beneath the world's diversities there was a common body of faith, a "natural religion", universally present in the human heart. In 1624, just when Grotius was preparing his case for natural law, Lord Herbert of Cherbury in his De veritate gave a description of natural religion, which was to pass substantially unchanged into Thomas Paine's Age of Reason (1794). Certain beliefs, said Herbert, are so manifest that all reasonable men accept them: the existence of one God who dispenses rewards and punishments, and the obligation resting on men to worship him in repentant piety and virtue. These articles suffice for a religious life in the present and for salvation in the hereafter.

    For nearly two centuries the ever-widening empire of scientific rationalism sustained the argument. Reasoning from a mathematically ordered nature to its Architect, the protagonists of natural religion, the deists, allowed the Architect only those qualities manifest in his handiwork. A widespread battle with Christianity ensued. The deists turned their weapons against the gamut of special beliefs and practices of Christianity, but they pressed most relentlessly against miracles. The marvel, contended the deists, lay in the order and the goodness of the divine creation: it was unthinkable that God would, or could, set aside laws expressive of his own being.

    Deism was never an organised movement. Its first home was among English intellectuals, but widening tolerance in England took away the incitement to controversy, so that the issues steadily lost in public interest. In France, on the other hand, the rigid structure of the ancien régime provoked angry resentments and Voltaire, aided by numerous lesser philosophers, used deism as a rapier for piercing the clerical side of the body politic. The rationalist invasion was less heralded elsewhere, but it persisted during most of the 18^th century. There was no census of converts, but certainly deism strongly influenced the upper strata of society all over Europe and in the American colonies.

    Christianity's first methods of self-defense were the traditional ones of dogmatic assertion of its truth and legal punishment, but neither proved effective. In a rationalist age supporters of the Christian faith increasingly had to admit the validity of natural religion and to search out reasonable grounds for adding revelation. In the second half of the 17^th century John Tillotson, who died archbishop of Canterbury, spoke for many when he held that Christianity was needed to complete the findings of natural religion: its service, he explained, was to test the reasoning process and to give a supplementary motive for a moral life. Tillotson conceded much, but his contemporary John Locke even more when he accepted reason as a judge of the truth of things "above Reason." A generation later, in his Analogy (1736), Joseph Butler, himself later a bishop, presented a long-famous restatement of the rationalist defense, but his final contention was that in natural religion and nature there were things just as difficult to believe as was Christian doctrine.

    While deists and Christians exchanged volleys, dangers were in preparation for both. One was a revival of materialism inspired by the mechanistic explanations of modern science. Francis Bacon leaned toward it and Thomas Hobbes embraced it thoroughgoingly. In France at about the same time Descartes tried to save spiritual values by a theory of dualism, but his mechanically operated material world gave encouragement to bolder men who followed. By 1750 the bars against extreme opinions had been somewhat lowered and several materialists ventured out into the open. Julien Offray de Lamettrie proclaimed that all things were reducible to matter in motion and he defiantly entitled one of his books L'Homme machine. The baron d'Holbach insisted that the cosmos was only matter without freedom or immortality or even first cause, and he looked on religion as the main source of human degradation. Few, however, were prepared to accept such extravagances.

    A more subtle danger for Christianity appeared in the scepticism of David Hume. Dissecting with amiable reasonableness, Hume cut away all certainty in human affairs: the soul of man, the powers of reason, the existence of God, cause and effect -all the old anchors tore loose in a great storm of doubt. Immanuel Kant tried to save reason and God from the wreckage, but he was able to do so only by setting radical limits to the domain of reason and by rediscovering God through a nonrational course.

    Meanwhile there had been an even more important response to the arguments questioning Christian belief. Religion, it was asserted, did not need to submit to a test of rationality. The mystics and others of unassailable faith went their separate ways in reaffirmed belief. Blaise Pascal, on the other hand, felt agonisingly the tension between the old and the new but found the answer in this wager on the existence of God and his cry: "The heart has its reasons which reason does not know." A too disputatious theology, with an overintellectualized and dehumanised deity, provided little spiritual nourishment, and a reaction was inevitable. Indeed, at the high noon of rationalism John Wesley brought great masses of Enlightenment back to engrossing concerns of salvation through Christ. At the same time Jean Jacques Rousseau preached his own sermons on the religion of the heart. The attempt, therefore, to apply natural-law rationalism to provoke a rebellion, which led to rationalism's defeat.

    Before Bacon's death in 1626 an even more explicit comparison between the classical world and the modern began to take shape. At issue were the relative merits of the second literature to those bred in reverence for Homer and Virgil it was sacrilege when certain critics preferred authors of their own time. For a whole century, Europe resounded with the "warfare of the ancients and the moderns". The battle raged on the subjective field of taste, and a clear-cut victory was impossible. When in 1683, however, B. Le Bovier de Fontenelle shifted the ground from literature to science, few could deny the reality of progress. But Fontanelle, like Bacon, was primarily concerned with the future, and he detected a law of progress determining its destiny.