. . . when we come to inspect the watch, we perceive. . . that its several parts are framed and put together for a purpose, e.g. that they are so formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day; that if the different parts had been differently shaped from what they are, or placed after any other manner or in any other order than that in which they are placed, either no motion at all would have been carried on in the machine, or none which would have answered the use that is now served by it. . . . the inference we think is inevitable, that the watch must have had a maker -- that there must have existed, at some time and at some place or other, an artificer or artificers who formed it for the purpose which we find it actually to answer, who comprehended its construction and designed its use.¹
According to The Power of Logic (2009), my textbook for formal logic, it describes formal fallacy as "... an error in reasoning that involves the explicit use of an invalid form" (147). The invalid form that Paley uses is known as the fallacy of affirming the consequent. This invalid argument form can be summarized thus:
1. If A, then B.
So, 3. A.
This argument is fallacious because it asserts an unwarranted, unsupported conclusion merely because the person assumes that if the effect exists, the cause automatically exists. In order to understand this fallacy in respect to intelligent design, I will symbolize the argument as it is commonly seen in religious contexts:
N = Nature is designed by God, D = Things have design in nature,
1. If N, then D.
So, 3. N.
As we can see, saying that nature is designed by God is a fallacious assertion in this argument. The assertion comes out of thin air.
¹ Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature (1802).