Thus we have a problem. What do we mean when we say that a person could have done otherwise? Chisholm begins by introducing some concepts that will serve as conceptual primitives in his account. The first of these is a concept that Chisholm calls “causal contribution”. If one event helps to bring about another, then we have one familiar sort of causal contribution. But in other cases it is not an event, but a person, who causally contributes to some event (1966, 284). A case (if there are any) in which a person causally contributes to some event would be described as an instance of “ agent causation ”. Chisholm claims that in an earlier era philosophers would have thought that agent causation is the familiar notion; they would have assumed that event causation is to be explained in terms of it. He recognized that at the time of his writing the priority relation between the concepts had probably reversed.