 Scotus begins by arguing that there is a first agent (a being that is first in efficient causality). Consider first the distinction between essentially ordered causes and accidentally ordered causes. In an accidentally ordered series, the fact that a given member of that series is itself caused is accidental to that member’s own causal activity. For example, Grandpa A generates a son, Dad B, who in turn generates a son of his own, Grandson C. B’s generating C in no way depends on A—A could be long dead by the time B starts having children. The fact that B was caused by A is irrelevant to B’s own causal activity. That’s how an accidentally ordered series of causes works.