William Paley’s Natural Theology, the book by which he has become best known to posterity, is a sustained argument explaining the obvious design of humans and their parts, as well as the design of all sorts of organisms, in themselves and in their relations to one another and to their environment. Paley’s keystone claim is that “there cannot be design without a designer; contrivance, without a contriver; order, without choice;…means suitable to an end, and executing their office in accomplishing that end, without the end ever having been contemplated.” His book has chapters dedicated to the complex design of the human eye; to the human frame, which, he argues, displays a precise mechanical arrangement of bones, cartilage, and joints; to the circulation of the blood and the disposition of blood vessels; to the comparative anatomy of humans and animals; to the digestive system, kidneys, urethra, and bladder; to the wings of birds and the fins of fish; and much more. For more than 300 pages, Paley conveys extensive and accurate biological knowledge in such detail and precision as was available in 1802, the year of the book’s publication. After his meticulous description of each biological object or process, Paley draws again and again the same conclusion—only an omniscient and omnipotent deity could account for these marvels and for the enormous diversity of inventions that they entail.

On the example of the human eye he wrote:

I know no better method of introducing so large a subject, than that of comparing…an eye, for example, with a telescope. As far as the examination of the instrument goes, there is precisely the same proof that the eye was made for vision, as there is that the telescope was made for assisting it. They are made upon the same principles; both being adjusted to the laws by which the transmission and refraction of rays of light are regulated.…For instance, these laws require, in order to produce the same effect, that the rays of light, in passing from water into the eye, should be refracted by a more convex surface than when it passes out of air into the eye. Accordingly we find that the eye of a fish, in that part of it called the crystalline lens, is much rounder than the eye of terrestrial animals. What plainer manifestation of design can there be than this difference? What could a mathematical instrument maker have done more to show his knowledge of [t]his principle, his application of that knowledge, his suiting of his means to his end…to testify counsel, choice, consideration, purpose?

It would be absurd to suppose, he argued, that by mere chance the eye

should have consisted, first, of a series of transparent lenses—very different, by the by, even in their substance, from the opaque materials of which the rest of the body is, in general at least, composed, and with which the whole of its surface, this single portion of it excepted, is covered: secondly, of a black cloth or canvas—the only membrane in the body which is black—spread out behind these lenses, so as to receive the image formed by pencils of light transmitted through them; and placed at the precise geometrical distance at which, and at which alone, a distinct image could be formed, namely, at the concourse of the refracted rays: thirdly, of a large nerve communicating between this membrane and the brain; without which, the action of light upon the membrane, however modified by the organ, would be lost to the purposes of sensation.

The strength of the argument against chance derived, according to Paley, from a notion that he named relation and that later authors would term irreducible complexity. Paley wrote:

When several different parts contribute to one effect, or, which is the same thing, when an effect is produced by the joint action of different instruments, the fitness of such parts or instruments to one another for the purpose of producing, by their united action, the effect, is what I call relation; and wherever this is observed in the works of nature or of man, it appears to me to carry along with it decisive evidence of understanding, intention, art…all depending upon the motions within, all upon the system of intermediate actions.

Natural Theology was part of the canon at Cambridge for half a century after Paley’s death. It thus was read by Darwin, who was an undergraduate student there between 1827 and 1831, with profit and “much delight.” Darwin was mindful of Paley’s relation argument when in the Origin of Species he stated: “If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.…We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind.”

In the 1990s several authors revived the argument from design. The proposition, once again, was that living beings manifest “intelligent design”—they are so diverse and complicated that they can be explained not as the outcome of natural processes but only as products of an “intelligent designer.” Some authors clearly equated this entity with the omnipotent God of Christianity and other monotheistic religions. Others, because they wished to see the theory of intelligent design taught in schools as an alternate to the theory of evolution, avoided all explicit reference to God in order to maintain the separation between religion and state.

The call for an intelligent designer is predicated on the existence of irreducible complexity in organisms. In Michael Behe’s book Darwin’s Black Box: The Biochemical Challenge to Evolution (1996), an irreducibly complex system is defined as being “composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.” Contemporary intelligent-design proponents have argued that irreducibly complex systems cannot be the outcome of evolution. According to Behe, “Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.” In other words, unless all parts of the eye come simultaneously into existence, the eye cannot function; it does not benefit a precursor organism to have just a retina, or a lens, if the other parts are lacking. The human eye, they conclude, could not have evolved one small step at a time, in the piecemeal manner by which natural selection works.

The theory of intelligent design has encountered many critics, not only among evolutionary scientists but also among theologians and religious authors. Evolutionists point out that organs and other components of living beings are not irreducibly complex—they do not come about suddenly, or in one fell swoop. The human eye did not appear suddenly in all its present complexity. Its formation required the integration of many genetic units, each improving the performance of preexisting, functionally less-perfect eyes. About 700 million years ago, the ancestors of today’s vertebrates already had organs sensitive to light. Mere perception of light—and, later, various levels of vision ability—were beneficial to these organisms living in environments pervaded by sunlight. As is discussed more fully below in the section Diversity and extinction, different kinds of eyes have independently evolved at least 40 times in animals, which exhibit a full range, from very uncomplicated modifications that allow individual cells or simple animals to perceive the direction of light to the sophisticated vertebrate eye, passing through all sorts of organs intermediate in complexity. Evolutionists have shown that the examples of irreducibly complex systems cited by intelligent-design theorists—such as the biochemical mechanism of blood clotting (see coagulation) or the molecular rotary motor, called the flagellum, by which bacterial cells move—are not irreducible at all; rather, less-complex versions of the same systems can be found in today’s organisms.

Evolutionists have pointed out as well that imperfections and defects pervade the living world. In the human eye, for example, the visual nerve fibres in the eye converge on an area of the retina to form the optic nerve and thus create a blind spot; squids and octopuses do not have this defect. Defective design seems incompatible with an omnipotent intelligent designer. Anticipating this criticism, Paley responded that “apparent blemishes…ought to be referred to some cause, though we be ignorant of it.” Modern intelligent-design theorists have made similar assertions; according to Behe, “The argument from imperfection overlooks the possibility that the designer might have multiple motives, with engineering excellence oftentimes relegated to a secondary role.” This statement, evolutionists have responded, may have theological validity, but it destroys intelligent design as a scientific hypothesis, because it provides it with an empirically impenetrable shield against predictions of how “intelligent” or “perfect” a design will be. Science tests its hypotheses by observing whether predictions derived from them are the case in the observable world. A hypothesis that cannot be tested empirically—that is, by observation or experiment—is not scientific. The implication of this line of reasoning for U.S. public schools has been recognized not only by scientists but also by nonscientists, including politicians and policy makers. The liberal U.S. senator Edward Kennedy wrote in 2002 that “intelligent design is not a genuine scientific theory and, therefore, has no place in the curriculum of our nation’s public school science classes.”

Scientists, moreover, have pointed out that not only do imperfections exist but so do dysfunctions, blunders, oddities, and cruelties prevail in the world of life. For this reason theologians and religious authors have criticized the theory of intelligent design, because it leads to conclusions about the nature of the designer at odds with the omniscience, omnipotence, and omnibenevolence that they, like Paley, identify as the attributes of the Creator. One example of a “blunder” is the human jaw, which for its size has too many teeth; the third molars, or wisdom teeth, often become impacted and need to be removed. Whereas many people would find it awkward, to say the least, to attribute to God a design that a capable human engineer would not even wish to claim, evolution gives a good account of this imperfection. As brain size increased over time in human ancestors, the concurrent remodeling of the skull entailed a reduction of the jaw so that the head of the fetus would continue to fit through the birth canal of the adult female. Evolution responds to an organism’s needs not by optimal design but by tinkering, as it were—by slowly modifying existing structures through natural selection. Despite the modifications to the human jaw, the woman’s birth canal remains much too narrow for easy passage of the fetal head, and many thousands of babies die during delivery as a result. Science makes this understandable as a consequence of the evolutionary enlargement of the human brain; females of other animals do not experience this difficulty.

The world of life abounds in “cruel” behaviours. Numerous predators eat their prey alive; parasites destroy their living hosts from within; in many species of spiders and insects, the females devour their mates. Religious scholars in the past had struggled with such dysfunction and cruelty because they were difficult to explain by God’s design. Evolution, in one respect, came to their rescue. A contemporary Protestant theologian called Darwin the “disguised friend,” and a Roman Catholic theologian wrote of “Darwin’s gift to theology.” Both were acknowledging the irony that the theory of evolution, which at first had seemed to remove the need for God in the world, now was convincingly removing the need to explain the world’s imperfections as outcomes of God’s design.