For these voluntarist, existentialist, and experiential conceptions of faith the place of reason in religion, although important, is secondary. Reason cannot directly establish the truth of religious propositions, but it can defend the propriety of trusting one’s deeper intuitions or one’s religious experience and basing one’s beliefs and life upon them. These schools of thought assume that the philosophical arguments for and against the existence of God are inconclusive, and that the universe is capable of being consistently thought of and experienced in both religious and naturalistic ways. This assumption, however, runs counter to the long tradition of natural theology. Christian philosophy as natural theology
Natural theology is generally characterized as the attempt to establish religious truths by rational argument and without reliance upon alleged revelations. It has focused traditionally on the topics of the existence of God and the immortality of the soul. Arguments for the existence of God The design (or teleological) argument
St. Paul, and many others in the Greco-Roman world, believed that the existence of God is evident from the appearances of nature: “Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made” (Romans 1:20). The most popular, because the most accessible, of the theistic arguments is that which infers a divine designer from perceived evidence of design in nature. The argument, propounded by medieval Christian thinkers, was developed in great detail in 17th- and 18th-century Europe by writers such as Robert Boyle, John Ray, Samuel Clarke, and William Derham and at the beginning of the 19th century by William Paley. They asked: Is not the eye as manifestly designed for seeing, and the ear for hearing, as a pen for writing or a clock for telling the time; and does not such design imply a designer? The belief that the universe is a coherent and efficiently functioning system likewise, in this view, indicates a divine intelligence behind it.
The argument from design was criticized by the Scottish philosopher David Hume in his Dialogues Concerning Natural Religion (1779). Hume conceded that the world constitutes a more or less smoothly functioning system; indeed, he points out, it could not exist otherwise. He suggests, however, that this may have come about as a result of the chance permutations of particles falling into a temporary or permanent self-sustaining order, which thus has the appearance of design. A century later the idea of order without design was rendered more plausible by Charles Darwin’s discovery that the adaptations of the forms of life are a result of the natural selection of inherited characteristics having positive, and the elimination of those having negative, survival value within a changing environment. Hume also pointed out that, even if one could infer an intelligent designer of the world, one would not thereby be entitled to claim that such a designer is the infinitely good and powerful Creator who is the object of Christian faith. For the world is apparently imperfect, containing many inbuilt occasions of pain and suffering, and one cannot legitimately infer a greater perfection in the cause than is observed in the effect.
In the 20th century, however, the design argument was reformulated in more comprehensive ways, particularly by the British philosophers Frederick R. Tennant (Philosophical Theology, 1928–30) and Richard Swinburne (using Thomas Bayes’s probability theorem in The Existence of God, 1979), taking account not only of the order and functioning of nature but also of the “fit” between human intelligence and the universe, whereby humans can understand its workings, as well as human aesthetic, moral, and religious experience. There were also attempts to show that the evolution of the universe, from the “big bang” of 13.8 billion years ago to the present state that includes conscious life, required the conjunction of so many individually improbable factors as to be inexplicable except as the result of a deliberate coordinating control. If, for example, the initial heat of the expanding universe, or its total mass, or the strength of the force of gravity, or the mass of neutrinos, or the strength of the strong nuclear force, had been different by a small margin, there would have been no galaxies, no stars, no planets, and hence no life. Surely, it was argued, all this must be the work of God creating the conditions for human existence.
These probability arguments were, however, strongly criticized. A basic consideration relevant to them all is that there is by definition only one universe, and it is difficult to see how its existence, either with or without God, can be assessed as having a specific degree of probability in any objective sense. It can of course be said that any form in which the universe might be is statistically enormously improbable, as it is only one of a virtual infinity of possible forms. But its actual form is no more improbable, in this sense, than innumerable others. It is only the fact that humans are part of it that makes it seem so special, requiring a transcendent explanation. Debate about the design argument continued through the late 20th and early 21st centuries, particularly in the United States (see also intelligent design). The cosmological argument
Aquinas gave the first-cause argument and the argument from contingency—both forms of cosmological reasoning—a central place for many centuries in the Christian enterprise of natural theology. (Similar arguments also appeared in parallel strands of Islamic philosophy.) Thomas’s formulations (Summa theologiae, I, Q. 2, art. 3) were refined in modern neo-Thomist discussions and remained topics of Christian philosophical reflection during the 20th century.
The first-cause argument begins with the assumption that there is change in the world. Change is always the effect of some cause or causes. Each cause is itself the effect of a further cause or set of causes; this chain moves in a series that either never ends or is completed by a first cause, which must be of a radically different nature in that it is not itself caused. Such a first cause is an important aspect, though not the entirety, of what Christianity means by God.
Although taking a different route, the argument from contingency follows the same basic movement of thought from the nature of the world to its ultimate ground. It starts with the premise that everything in the world is contingent for its existence upon other factors. Its presence is thus not self-explanatory but can only be understood by reference beyond itself to prior or wider circumstances that have brought it about. These other circumstances are also contingent, pointing beyond themselves for the ground of their intelligibility. If this explanatory regress is unending, explanation is perpetually postponed and nothing is finally explained. The existence of anything and everything thus remains ultimately unintelligible. But rational beings are committed to the search for intelligibility and cannot rest content until it is found. The universe can only finally be intelligible as the creation of an ontologically necessary being who is eternal and whose existence is not contingent upon anything else. This is also part of what Christianity has meant by God.