Imagine you are walking on a mildly populated sidewalk downtown one afternoon. You walk from your office, to the elevator, then out the doors, along Park Street, turn onto Haven Street, and finally reach Bends (on step #N), a local sub shop. It is a logical necessity, that at every point along your way, you traversed every step along your way. That is to say, each step is an integral part of the event series which brought you from your cubicle to Bends. If, ex hypothesi, “your taking a 99nd step as you walk to Bends” were magically erased from the universe of facts, then when you take step #98, you will stop, never reaching Bends. Even so, is it true to say that the steps you took along the way were what caused you to move along the way? Is it true to say that step 98# caused step 99#?
Imagine further that, in this semi-magical universe (or, for a more mundane reason that I’ll stipulate in a moment), just as you are a few steps away from entering Bends, it is suddenly transported across the street. Whereupon, as an accustomed inhabitant of such a universe, you turn on your heel and cross the street to the relocated but happily still nearby Bends. (The more mundane version is that you were under the impression that Bends was at the corner of Haven and Plainview, but in fact it is across the street. Once you see your mistake, you change course.) In this case, can we really say there was something about the physical constitution of your (N-1)th step and its efficent-causal power which caused you to end up at Bends?
On the one hand we must admit that if any or all of the steps were “erased”, you would not reach Bends. On the other hand, we cannot reasonably claim any one of the steps caused any of the others, and therefore neither that the steps themselves caused your arrival. As a larger truth, constituent events (such as the steps) which take place in a whole action are causally subsidiary to the whole event.
[I anticipate two classes of objections, but I will stop here in order to see what the cat drags in. Then, if the objections I have in mind do show up, or perhaps others I don’t foresee, I’ll continue this post.]