 Is what is true of the past necessary? Well certainly pretty well everyone thinks that what happened in the past cannot be undone. The past cannot now be altered. For if one were to undo the past, that would mean that something which did happen did not happen; or, at the very least, that it was true that something happened and then later was not true that it happened. Most people (but not such a large majority) also think that something else is impossible, namely affecting what happened in the past or causing something to have happened in the past. (This is not the same thing, of course, as altering the past. If one caused something to have happened, one would not thereby bring it about that something that did not happen did happen.) But it is one thing to think that one cannot make it true that there was a sea-battle yesterday, and quite another thing to think that one cannot make it true yesterday that there will be a sea-battle tomorrow.