St. Thomas Aquinas (1225–1274) made use of five “ways” for the existence of God. The second of these five ways — or arguments — was the “first cause” argument. Aquinas wrote: “The second way is from the nature of efficient cause. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.” Imagine that you look outside your window and see a tree branch swaying. That branch is being moved by the wind. That wind has its causes, and so on. But as with motions, you cannot go on to infinity in a series of causes. If there is no first cause, then there will be no intermediate causes, like the wind and the swaying tree branches that result from it. “Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God,” Aquinas concludes.

Russell followed the reasoning through, writing, “If everything must have a cause, then god must have a cause. If there can be anything without a cause, it may just as well be the world as god, so there cannot be any validity in that argument. It is exactly of the same nature as the Hindu's view, that the world rested upon an elephant and the elephant rested upon a tortoise; and when they said, `How about the tortoise?' the Hindu said, ‘Suppose we change the subject.’ The argument is really no better than that.”