 Well, one might certainly object to presupposition 6, on the grounds that it does indeed seem to be the passage of time which makes a difference to my power to bring about or prevent a sea-battle on a certain day. Till the day is over, I may have the power, but after the day, I have not. However, presupposition 6 does not in fact seem to play a significant role in the parallel arguments. So that cannot be the whole story.