 For Mackie (1974: 85–86), this type of problem indicates that his regularity theory is at best incomplete. And he tells us what is missing: a genuine cause-effect relation is marked by causal priority. As an approximation, a token event \(c\) was causally prior to another token event \(e\) if an agent could have—in principle—prevented \(e\) by “(directly or indirectly) preventing, or failing to bring about or to do \(c\)” (1974: 190). To prevent the sounding of the Manchester hooters does not prevent the workers from going home in London. Hence, the instance of \(E_1\) was not causally prior to the associated instance of \(E_2\) (though it is temporally prior). By contrast, preventing the sounding of the London hooters does prevent the workers from going home in London.