 But suppose, his argument continues, we let O and O′ be the act of ordering a sea-battle and the act of ordering no sea-battle, and Q and Q′ be the propositions that there will and will not be such a battle; and we substitute O and O′ for S and S′, and Q and Q′ for P and P′, and “tomorrow” for “yesterday” in the argument above, then (if we make some obvious assumptions about the relation between what we order and what happens) we have a parallel argument which goes: 