Okay, I can see that I need to spell out in more detail one of the ideas here - namely that you're trying to generalize over a repeating type of causal link and that reference is pinned down by such generalization. The Sun repeatedly sends out light in individual Sun-events, electrons repeatedly go on traveling through space instead of vanishing; in a universe like ours, rather than the F(i) being whole new transition tables randomly generated each time, you see the same F(physics) over and over. This is what you can pin down and refer to. Any causal graph is acyclic and can be divided as you say; the surprising thing is that there are no F-types, no causal-link-types, which (over repeating time) descend from one kind of variable to another, without (over time) there being arrows also going back from that kind to the other. Yes, we're generalizing and inducting over time, otherwise it would make no sense to speak of thingies that "affect each other". No two individual events ever affect each other!