Lecture: IBE and Paley’s Watch
Although there were plenty of versions of this style of argument, William Paley’s (1743–1805) was and still is very influential. Darwin is said to have brought his copy of Natural Theology—where the passage comes from—along with him on his five-year voyage on the Beagle.
Unlike St. Anselm’s and St. Aquinas’s deductive arguments, this one is nondeductive. Whether it is an argument by induction, analogy, or inference to the best explanation is slightly controversial among philosophers. In what follows I’ll impose my own views on this—that Paley’s argument is indeed an inference to the best explanation. I’ll make the case in a rather lengthy discussion (I have adapted the discussion from a paper I wrote: Ariew, A. “Teleology.” In M. Ruse and D. Hull’s Cambridge Companion to Philosophy of Biology. Cambridge University Press. 2008.)
Paley’s argument starts with an analogy between living organisms and human artifacts. Briefly, if you come across a watch and inquire as to its existence, you would not take seriously the conclusion that watches are the product of natural forces. It is highly improbable that natural forces would randomly coalesce matter into a watch.
The possible existence of a designer who can manipulate the parts for his or her own purpose makes the existence of watches much more plausible. Paley’s conclusion is that the existence of a designer best explains watches and living organisms.
As I said above, in my view, Paley’s is an instance of an inference to the best explanation. Paley’s inference to the existence of watchmakers (and later, his argument for the existence of a divine creator) works exactly in this way. The existence of watchmakers is supported by the existence of watches because watchmakers best explain how such complex things could come to exist.
An interesting feature of inference to the best explanation arguments is that with them, we can infer the existence of unobservable phenomena. As Paley put it, already having inferred the existence of a watchmaker from inspection of a watch found on a dirt path:
Nor would it, I apprehend, weaken the conclusion, that we had never seen a watch made—that we had never known an artist capable of making one—that we were altogether incapable of executing such a piece of workmanship ourselves, or of understanding in what manner it was performed.
The strength of the inference to a watchmaker depends not upon our witnessing watchmakers making watches but in the relative likelihood that watches would exist if skilled watchmakers were to exist. Compare that to the likelihood of the materialist hypothesis that watches would exist only if the random action of natural forces were to exist. Natural processes alone are unlikely to make a watch. A creator with forethought more likely will.
The feature of inferring the existence of unobservable causes distinguishes inferences to the best explanation from garden-variety inductive arguments. Let’s see why. The strength of an inductive argument depends on the size and bias of the sample. Yet, as Paley suggests in the quote above, we may never have seen watchmakers make watches. If so, our sample size is zero—likewise for the sample of times in which any of us has seen God creating living things. If teleological arguments are inductively based, then they would be nonstarters. It’s a good thing for Paley that his inference is not inductive. In an inference to the best explanation, the issue is not the properties of a sample but rather what could explain an observed phenomenon.
Next, Paley considers what would happen if we found a self-replicating watch. The passage is a lovely early example of science fiction:
Suppose, in the next place, that the person who found the watch should after some time discover that, in addition to all the properties which he had hitherto observed in it, it possessed the unexpected property of producing in the course of its movement another watch like itself—the thing is conceivable; that it contained within it a mechanism, a system of parts—a mold, for instance, or a complex adjustment of lathes, files, and other tools—evidently and separately calculated for this purpose; let us inquire what effect ought such a discovery to have upon his former conclusion.
In addition to serving the function of telling time, this watch has a further extraordinary feature: it produces well-functioning offspring. The discovery of a self-replicating watch affects the former conclusion—the existence of watchmakers—in several important ways. First, it further illustrates that the strength of Paley’s inference does not depend on ever having seen any watchmakers make watches. As I argued above, this feature distinguishes inference to the best explanation arguments from inductive ones because it does not depend on sampling from a population of events.
Second, and more important, the discovery of the self-replicating watch strengthens the inference to the existence of a designer at the same time that it weakens the inference to the hypothesis that the item is the product of natural forces alone. As Paley puts it, “If that construction without this property, or, which is the same thing, before this property had been noticed, proved intention and art to have been employed about it, still more strong would the proof appear when he came to the knowledge of this further property, the crown and perfection of all the rest.” The probability of natural forces randomly producing a watch is very small, but the probability of natural forces randomly producing something as extraordinary and exquisite as a self-replicating watch is even smaller.
The general lesson is this: the more complex the parts, the stronger the evidence of a designer. It and the next feature play a large role in Paley’s ultimate inference, the existence of a God.
The third effect that the self-replicating watch example has on the former conclusion is, in Paley’s words, to “increase his admiration of the contrivance, and his conviction of the consummate skill of the contriver.” However complex watches are, most of us with average intelligence and skills could imagine learning, after extensive training, how to create watches. But to have the skill of a self-replicating watchmaker would be extraordinary or even supernatural. The general lesson here is that the more complex the design, the more intelligent or skillful the designer.
Once the second and third features of the new inference to the best explanation from self-replicating watches are in place, all that Paley has to convince us of is that living tissues, organs, organisms, and ecosystems are much more complex than self-replicating watches and that their parts are much more attuned to the functions that they serve. In a sense, that is the intention of the bulk of Paley’s book, Natural Theology, from which his famous argument for the existence of an intelligent designer is a relatively small section. The later chapters are more or less a zoological textbook, detailing the wonder of natural adaptations. The most famous passages are found in the section that describes the anatomy and function of the eye. For instance, he expresses amazement at how the anatomy of eyes from animals living in distinct environments differs according to the laws of transmission and refraction of rays of light. I’ll let Paley express the point himself (this is from later on in Paley’s book):
For instance, these laws require, in order to produce the same effect, that 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 his principle, his application of that knowledge, his suiting of his means to his end—I will not say to display the compass or excellence of his skill and art, for in these all comparison is indecorous, but to testify counsel, choice, consideration, purpose?
Passages like this remind me of the many nature programs found on the Discovery channel (http://dsc.discovery.com/convergence/planet-earth/planet-earth.html) or PBS (http://www.pbs.org/lifeofbirds/) that showcase nature’s great adaptations. One would think, given the shows’ sponsorships, that the purpose of these shows is to educate the public about the wonders of a biological world that emerges from natural selection. But ironically, as passages like the one above from Paley remind us, there’s no reason to think that the very same phenomena featured on these nature programs could not equally serve as grist for the Creationists’ mill.
Does Paley’s argument succeed in proving that living organisms are created by God? Among his contemporaries, he provided a powerful argument against a materialist who seems to have little to account for natural adaptation. Yet, the success of inference to the best explanation arguments depend on the relative success of the given hypotheses, and, as Darwin would show, there are other hypotheses to consider besides the random action of matter and cause and an intelligent designer. Therein lies a formal limitation of inferences to the best explanation. The strength of an inference to the best explanation is only as good as the proffered hypotheses. For any given set of hypotheses, the interlocutor has always the option to remain agnostic as to the cause of the phenomenon in question. To suggest otherwise—for instance, to argue that because God is a better explanation than matter and cause, God must exist—is to commit “the only game in town” fallacy. As for proving the existence of an omnipotent and omniscient God, Paley’s commentators and critics often pointed out that while Paley’s inference might have strongly suggested the existence of a supremely intelligent designer (a supernatural designer), it stops short of proving that the designer is God.