 Let “now-necessary” designate temporal necessity, the type of necessity that the past is supposed to have just because it is past. This type of necessity plays a central role in the argument and we’ll have more to say about it in sections  2.4, 2.5, 2.7,  and  5,  but we can begin with the intuitive idea that there is a kind of necessity that a proposition has now when the content of the proposition is about something that occurred in the past. To say that it is now-necessary that milk has been spilled is to say nobody can do anything now about the fact that the milk has been spilled.