## Root claim of the Five Ways

The central insight developed in the Five Ways is that causal chains are finite, where “chain” is a metaphor for any homogenous repetition of the same sort of thing. Like any homogeneous repetition there is no intrinsic limitation on the number of iterations that might be made, and so part of calling them “chains” is the recognition that we can imagine them going on forever.

So causal chains are on the one hand infinite since we can iterate homogeneous causes forever and they are finite since such iterations cannot explain the causal work being done. All the Five Ways turn on recognizing that causal chains are finite as causal and infinite as chains. “Infinite” means only that there is no intrinsic limitation on how many iterations of homogeneous causes one might make, not that the series necessarily goes on forever. There is no limit to how large you might make the Rube Goldberg machine that ends up accounting for some event or effect.

The central claim is not that the causal series has indeterminate length, but must be of some finite length, e.g. the Rube Goldberg Machine can get as big as you want but it must start somewhere. To understand the claim in this way requires more information than the claim itself can give. None of the Five ways specifies where or how the first cause effects the secondary causes, but only that there must be some primary cause whose activity is not and cannot be interactive.

In the Five Ways STA applies the root claim to the four genera of causality and claims that the first cause in each case deserves to be called deus. He orders the five arguments from more known to less known, starting with the causes easiest to understand (and least causal) and working upward to the hardest causes to understand (and most causal). The First Way starts with mobiles in potential, i.e. with material causality. The Second Way starts with agent causality.  The Third and Forth Way concern the two ways in which formal causality occurs: either intrinsically as a constitutive feature of things, and so making them generable or necessary, or extrinsically and so serving as a paradigm or measure of their being or activity. The Fifth Way appeals to final causality, which is hardest to understand and can only be discovered by a science when it is laying the finishing touches on its explanation of things.