Natural Theology

William Paley (1743–1805) was an Anglican clergyman and moral philosopher whose Natural Theology, published in 1802, became the most famous expression of the design argument in English. The book completes a unified philosophical system begun in Paley's earlier works on moral philosophy and the evidences of Christianity, arguing for the existence and attributes of God through observation of the natural world.

The book opens with its celebrated thought experiment. Paley asks the reader to imagine crossing a heath and striking a foot against a stone. One might reasonably suppose the stone had always been there. But if one found a watch on the ground, the intricate arrangement of its springs, wheels, chains, and glass, all cooperating to mark the hour, would compel the conclusion that the watch had a maker. Paley systematically dismantles eight objections to this inference, including that no one has ever seen a watch made, that the watch sometimes malfunctions, and that some parts have unknown functions. He extends the analogy by imagining that the watch can produce another watch through its own mechanism, contending that this would only deepen admiration for the original contriver. Even an infinite chain of watches, each producing the next, would not eliminate the need for an original designing intelligence.

Paley applies this reasoning to the natural world, choosing the human eye as his primary example. He compares the eye to a telescope, arguing that both are constructed on identical optical principles for forming images through light refraction, but that the eye surpasses the telescope by automatically adjusting to varying degrees of light and accommodating objects at vastly different distances. He surveys the eyes of different species to show one general plan varied according to each creature's needs, and extends the same analysis to the ear. He argues that the production of offspring through reproduction does not account for the contrivance visible in organisms, just as a self-reproducing watch would not account for its own design: The parent is an instrument, not a designer.

Addressing broader objections, Paley distinguishes between the question of whether a Creator exists and the question of the Creator's attributes: imperfections in nature may bear on the latter but carry little weight against the former. He rejects the hypothesis that the present world is the surviving remnant of infinite random variations, dismisses appeals to a vague "principle of order" as a mere substitution of words for reasons, and insists that his argument is cumulative, with each example serving as an independent proof.

The middle portion of the book surveys mechanical contrivances in the animal body in extensive detail. Paley examines the vertebrae of the human neck, where a hinge joint allows nodding while a tenon-and-mortice arrangement allows rotation. He analyzes the two bones of the forearm, the spine's combination of firmness and flexibility, the oblique articulation of ribs for breathing, and the joints of the body, highlighting the ligament within the hip socket as one of the most unequivocal proofs of contrivance. Turning to muscles, he establishes that wherever a joint permits motion, muscles are positioned to produce that specific motion and no other, with opposing antagonist muscles enabling reciprocal movement. He traces the blood vessels and the four-chambered heart, the alimentary canal from mouth through intestines, and the species-specific nature of digestion.

Viewing the animal body as a whole, Paley marvels at its bilateral symmetry, the secure packaging of vital organs, and the beauty of external form. He devotes a chapter to comparative anatomy, arguing that a general plan pursued with variations for each species provides the strongest evidence of intelligence. He surveys animal coverings, bills, bones, and the mechanics of flight. A chapter on peculiar structures treats the viper's perforated fang, the air bladder that allows fish to regulate buoyancy, the opossum's pouch, the woodpecker's barbed tongue, and the camel's water-storing stomach sacs.

Paley examines what he calls prospective contrivances: provisions made in advance of their use. Human teeth form within the gums but their eruption is delayed during infancy, with a second set prepared beneath the first. The eye is constructed in the darkness of the womb for a world of light. The lungs lie collapsed in the fetal chest, ready for the instant breathing begins. Temporary bypasses in the fetal heart route blood around the non-functioning lungs and close after birth.

The concept of "relation," or the fitness of different parts to one another for a common effect, receives its own chapter, with detailed illustrations drawn from the swan and the mole. The chapter on compensation shows how defects in one part are supplied by another: the elephant's trunk compensates for its short neck, and birds without teeth compensate with the powerful gizzard. Paley extends his argument to the relationship between organisms and the inanimate elements, and treats instincts as propensities prior to experience, arguing that a hen's incubation of eggs and a butterfly's placement of eggs on the correct plant cannot be explained by reasoning or learning. Chapters on insects and plants survey further contrivances, from the compound sting of bees to the elaborate packaging and dispersal mechanisms of seeds.

A chapter on astronomy argues that the placement of light and heat at the center of the solar system, the earth's rotation on a permanent axis, the inverse-square law of gravitation, and the nearly circular planetary orbits all represent choices from among vast ranges of possibility that could not have occurred by chance. Paley acknowledges that astronomy is less suited than biology to proving design, since heavenly bodies appear too simple for the argument from the relation of parts, but contends that it demonstrates the magnificence of divine operations.

In the final chapters, Paley draws his theological conclusions. He argues that contrivance implies consciousness and thought, capacities that constitute personality and require a mind: "The marks of design are too strong to be got over. Design must have had a designer. That designer must have been a person. That person is God" (229). He criticizes the misapplication of the terms "law" and "mechanism" as explanatory causes, insisting that a law is only the mode according to which an agent proceeds and that mechanism without power can do nothing. He rejects the theory that organisms formed gradually through "appetencies," or innate propensities of matter, arguing that no transitional changes have ever been observed. He discusses the divine attributes in careful terms, argues for the unity of God from the uniformity of plan across the universe, and advances two propositions regarding divine goodness. First, the vast majority of observed contrivances are beneficial, and the world is overwhelmingly happy. Second, God has superadded pleasure to animal sensations beyond what survival requires: The sweetness of food, harmony in sound, fragrance in smell, and beauty in sight are gifts of pure benevolence. He addresses the origin of evil through the consideration of general laws whose crossings produce particular inconveniences and the view of life as a state of moral probation.

Paley concludes by affirming that habitual contemplation of nature with constant reference to an intelligent author transforms the world into a temple and life into "one continued act of adoration" (279). Natural theology, he contends, facilitates the articles of revealed religion, especially the resurrection of the dead, by demonstrating the existence of a power capable of arranging planetary systems and constructing the filaments of a hummingbird's feather.