This principle of dual causation was crucial for reconciling reason and revelation--Athens and Jerusalem--so that Jews, Muslims, and Christians could embrace Aristotle's natural philosophy as compatible with their religious belief in God as Creator. This reconciliation of Biblical religion and Aristotelian naturalism provided the cultural conditions for the emergence of modern science in the Western world.
The tension between religion and science continues to stir emotional debate today, particularly among those Christians and Muslims who worry about whether evolutionary science is compatible with their religious beliefs. The principle of dual causality is crucial today for those religious believers who want to reconcile biblical creationism and Darwinian evolution.
Darwin's novel contribution was in extending the principle of dual causation to allow for the origin of species and the emergence of human morality through the "secondary causes" of natural evolutionary laws.
In the concluding chapter of The Origin of Species, Darwin tried to persuade Christian creationists that it was a nobler conception of the Creator to see Him as working through the secondary causes of evolution rather than having to specially create each form of life by miraculous intervention.
To my mind it accords better with what we know of the laws impressed on matter by the Creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual. When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled.
In the sixth edition of the Origin, Darwin quoted from his friend the Reverend Charles Kingsley, who had written that he had "gradually learnt to see that it is just as noble a conception of the Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply the voids caused by the actions of His laws."
In his Autobiography, Darwin described "the extreme difficulty or rather impossibility of conceiving this immense and wonderful universe, including man with his capacity of looking far backwards and far into futurity, as the result of blind chance or necessity. When thus reflecting I feel compelled to look to a First Cause having an intelligent mind in some degree analogous to that of man; and I deserve to be called a Theist."
But then he indicated that since writing the Origin, he had begun to doubt this. He wondered whether the human tendency to see a necessary connection between cause and effect might be merely a product of inherited experience embedded by evolution in the human brain. Or it might be that the cultural tradition of belief in God, inculcated in the minds of children, has made it hard for many human beings to throw off their religious beliefs. "I cannot pretend to throw the least light on such abstruse problems," he observed. "The mystery of the beginning of all things is insoluble by us; and I for one must be content to remain an Agnostic."
Darwin was also concerned that if we believe that God has designed everything in the world down to the last detail, then we must hold Him responsible for all the evil in the world. He thought that evolution could explain the occurence of evil in a world in which God is not directly responsible for everything.
In arguing that it was "nobler" for the Creator as First Cause to confer on His creatures the dignity of acting as secondary causes, Darwin was continuing a tradition of thought stretching back to the Middle Ages. Some biblical believers assumed that the glory of God as the omnipotent Creator required that He should be the only cause, so that He would have no need for intermediary processes to carry out His will. But Aquinas and others argued that it denigrated God's goodness to say that He was not good enough or powerful enough to give creatures their own causal power. To allow creatures to exercise power as secondary causes conferred dignity on them and glory to the Creator. "To take away their proper actions from things is to disparage the divine goodness" (SCG, III, 69, 16).
Moreover, Aquinas insisted, this dual causality was the necessary condition for natural science: "if created things have no actions productive of effects, it follows that no nature of anything would ever be known through the effect. And thus, all the knowledge of natural science is taken away from us, for the demonstrations in it are chiefly derived from the effect" (SCG, III, 69, 18).
This Christian defense of natural science was controversial in the Middle Ages, because natural science was understood primarily as coming from the pagan natural philosophy of Aristotle. Up to the twelfth century, most of the writings of Plato and Aristotle were not available in the Christian West. Christians could read only Plato's Timaeus and Aristotle's logical works. The theological cosmology of the Timaeus was interpreted as resembling Biblical creationism. And, similarly, Aristotle's logical works posed no challenge to Christian beliefs. But then in the twelfth century, the Arabic translations of Plato and Aristotle were translated into Latin, and for the first time, Christians could read Aristotle's books of natural philosophy--including his biological works--for the first time. Since Aristotle's science did not recognize a Creator God or a supernatural realm, Aristotle's books of natural science were banned at the University of Paris and other Christian universities. Those promoting Aristotelian science were subject to punishment by the Inquisition. Even Aquinas himself was suspected of promoting heretical doctrines in teaching Aristotelian science. Despite this persecution, Aristotelian science prevailed and even became part of the regular curriculum at Christian universities, but only because Aquinas and others were able to persuade the Church authorities that Aristotle's account of nature and its causal laws could be understood as belonging to a realm of secondary causes, and thus compatible with belief in God as First Cause.
A similar debate had occurred earlier in the Islamic world. When the works of Plato and Aristotle were translated into Arabic by Muslim scholars in the eighth century, some Muslim theologians denounced this pagan knowledge as contrary to the revealed wisdom of the Koran. The Muslim proponents of Greek philosophy--particularly, Avicenna, Alfarabi, and Averroes--had to show how the natural philosophy or science of the Greeks was compatible with the Koran. Drawing on certain Platonic and Neo-Platonic texts, they developed the principle of dual causation, so that the study of the causal laws of nature could be seen as compatible with believing in God's omnipotence as First Cause.
Perhaps the earliest source of the idea of dual causation is in Plato's Timaeus (46d-e). This idea was further developed in some of the Neo-Platonic texts, and then concisely stated in The Book of Causes, an anonymous work written probably in the ninth or tenth century in Baghdad. When this book was translated into Latin in Toledo in the twelfth century, it became the primary source for this idea in the Christian West.
For at least 300 years, if not longer, science and philosophy flourished in the Islamic world. But then, beginning in the twelfth century, some Muslim theologians began to attack this intellectual movement as irreligious. The most influential of these was Abu Hamid Muhammad al-Ghazali, particularly in his work The Incoherence of the Philosophers. Representing the Asharite school of Islamic theology, al-Ghazali argued that God is the only cause, and therefore what human beings habitually regard as sequences of natural causes and effects are only contingent events that are arbitrarily decreed by God. Thus, there is no inherent necessity in the order of nature. To look for lawful causal necessities in nature--as scientists and philosophers do--is a blasphemous denial of God's unconstrained power.
Against al-Ghazali's Incoherence of the Philosophers, Averroes wrote his own book The Incoherence of the Incoherence to defend Greek science and philosophy as consistent with Muslim beliefs.
This debate continues today. Remarkably, the Muslim world today seems inhospitable to science, because so many Muslims assume that modern science is contrary to their religious beliefs. In many Muslim countries, the majority of believers reject evolutionary science as contradicting the creationism of the Koran. We tend to assume that the creation/evolution debate is mostly a product of fundamentalist Christian culture in the United States. But, in fact, this same debate is carried out in the Muslim world. A recent article in The Chronicle of Higher Education surveys this current debate over whether science--and particularly, evolutionary science--can be accepted by Muslim believers. It should be noted that some of the Muslim scientists defending evolution as compatible with the Koran use the same arguments that Averroes used against al-Ghazali, arguments that depend upon the principle of dual causation.
Similarly, among Christians in the Western World today, the interpretation of evolution as secondary causality continues to be the fundamental idea for justifying theistic evolution, which has been embraced by Christian scientists like Francis Collins, Christian writers like C. S. Lewis, and Christian politicians like Mitt Romney.
The religious appeal to God as the uncaused cause of nature cannot be refuted by reason. All natural explanations of the world--including Darwinian science--must assume that ultimately the order of nature is the unexplained ground of all explanation. But there is no way to deny the possibility that nature itself is the contingent product of nature's God.
Al-Ghazali, The Incoherence of the Philosophers, trans. Michael E. Marmura (Provo, UT: Brigham Young University, 2000).
The Book of Causes, trans. Dennis J. Brand (Milwaukee, WI: Marquette University Press, 1984).
Maurer, Armand, "Darwin, Thomists, and Secondary Causality," The Review of Metaphysics 57 (March 2004): 491-514.
Rubenstein, Richard, Aristotle's Children: How Christians, Muslims, and Jews Rediscovered Ancient Wisdom and Illuminated the Middle Ages (Orlando, FL: Harcourt, 2003).
Aquinas's reconciliation of Arabic and Greek science with Christian theology is beautifully depicted in Francesco Traini's altarpiece "Triumph of St. Thomas Aquinas", which is nicely reproduced as part of the cover for Rubenstein's book.
Some blog posts on related themes can be found here, here, here, here, here, here, here, here, here, here, here, here, here, here, here.