As we have seen, Paley’s argument appeals to an analogy between a watch- maker and the designer of the universe. That is why his argument is sometimes called the ‘analogy argument’. The analogy argument has the following generic structure:
1 The universe is similar to a watch.
2 A watch is created by an intelligent designer, in particular, a watchmaker. Therefore,
3 It is reasonable to conclude that the universe is also created by an intel- ligent designer.
This argument has the following more general structure: 1 x is similar to y.
2 x has property p (e.g., the property of being created by an intelligent designer).
Therefore,
3 It is reasonable to conclude that y also has property p (e.g., the property of being created by an intelligent designer).
Here is an example of another argument with the same structure: 1 A wolf is similar to a dog.
2 A wolf is a carnivore (i.e., a wolf has the property of being a carnivore). Therefore,
3 It is reasonable to conclude that a dog is also a carnivore (i.e., a dog also has the property of being a carnivore).
This argument is cogent. However, if an analogy is not appropriate, an argument of the same form could turn out to be uncogent. Consider, for example, the following argument:
1 A bicycle is similar to a tricycle.
2 A bicycle is two-wheeled (i.e., a bicycle has the property of being two-wheeled).
Therefore,
3 It is reasonable to conclude that a tricycle is also two-wheeled (i.e., a tricycle also has the property of being two-wheeled).
Although a bicycle and a tricycle are similar the conclusion does not follow from the two premises. What these examples teach us is that an analogy argument is cogent only if the two items compared are similar in an appro-
priate respect, not merely similar in any arbitrary respect. The above argument
about a tricycle is not cogent because a bicycle and a tricycle are not similar with respect to the number of wheels that they have, even though they are similar in other respects, such as the shape of their wheels and seats. So, in
order to make an analogy argument sound, the general structure presented above needs to be reformulated as follows:
1 x is similar to y in an appropriate respect. 2 x has property p.
Therefore,
3 It is reasonable to conclude that y also has property p. For example, the following argument satisfies the above schema: 1 A bicycle is similar to a tricycle with respect to its handlebars.
2 A bicycle has a handlebar with two grips (i.e., a bicycle has the property of having a handlebar with two grips).
Therefore,
3 It is reasonable to conclude that a tricycle also has a handlebar with two grips (i.e., a tricycle also has the property of having a handlebar with two grips).
This argument is cogent. It is not, however, always easy to know in what respect we can appropriately compare two things. Consider Paley’s argu- ment again. In what respect should we compare a watch and the universe? It cannot be with respect to the fact that they were both created by an intel- ligent designer because, if so, Paley has to presuppose that the universe was created by an intelligent designer even before constructing the argument. That would make his argument question-begging.
Hume says that there is no relevant sense of similarity here that allows us to derive the existence of an intelligent designer of the universe. In fact, there are many features of a watch and the universe that are dissimilar. For example, a watch is purely mechanical insofar as it contains no living tissue or parts but the universe, which contains organic matter, is not purely mechanical. To take another example, while the creation of a watch requires pre-existing material, such as gears, hands, glass, and so on, the creation of the universe does not. Many theists think that God created the universe
after the creation of the parts, but in the case of the universe, design and the creation of the parts seem to come simultaneously.
Hume also claims that the universe and a watch are dissimilar because, while the creation of the universe is a unique event, which has happened only once, the creation of ordinary objects, such as a watch and a house, has happened many times. When we see a watch we can infer that some watch- maker made it because we have seen many other watches and we know that all of them were built by watchmakers. That is, we make an inference of the following form:
1 We know that Watch 1 was created by a watchmaker. 2 We know that Watch 2 was created by a watchmaker. 3 We know that Watch 3 was created by a watchmaker. Therefore,
4 It is reasonable to conclude that every watch is created by a watchmaker. Therefore,
5 It is reasonable to conclude that the watch I have found on the ground was also created by a watchmaker.
The inference from premises (1), (2), and (3) to conclusion (4) is an instance of inductive reasoning; it derives a conclusion about watches in general from premises about particular watches. The inference from conclu- sion (4) to conclusion (5) is an instance of deductive reasoning because it derives a conclusion about a particular watch from a premise about watches in general.
What is unique about inductive reasoning is that the more observa- tions you have, the more robust your inductive reasoning becomes. Thus the above inductive reasoning about watches becomes more robust as we observe more watches and confirm that they are also created by watch- makers. Hume points out that we cannot construct an instance of strong inductive reasoning to derive the existence of an intelligent designer of the universe because we have never seen other universes that were created by intelligent designers. Paley’s inference would be robust if he could form the following inference:
1 We know that Universe 1 was created by an intelligent designer. 2 We know that Universe 2 was created by an intelligent designer. 3 We know that Universe 3 was created by an intelligent designer. Therefore,
4 It is reasonable to conclude that every universe is created by an intel- ligent designer.
Therefore,
5 It is reasonable to conclude that the universe in which we live was also created by an intelligent designer.
It is not possible, of course, to form such an inference because our universe is the only universe that we know.
Is contemporary intelligent design also vulnerable to these criticisms? It seems not. First, unlike Paley’s design argument, the intelligent design argu- ment does not rely on analogy, which could be subjective, in order to detect design. Instead it relies on properties that can arguably be detected objec- tively, such as irreducible complexity and specified complexity. Second, unlike Paley’s argument, the intelligent design argument does not focus on the whole universe. It focuses rather on a very specific biological system, such as the bacterial flagellum. Therefore, while the objection in question might refute Paley’s design argument it does not refute the contemporary intelligent design argument.