Krdytkyu

You look out of the window and it's raining. You grab a bottle and pour some water into a glass. You probably don't consider these events as necessary: it could have been a sunny day and you were free not to pour water into the glass. If you open a box and find a book inside it, you don't think that your act has affected the content, but just that the box contained a book and you didn't know it.

These two cases, however, are not so different: if it's not possible to change the past (take this a premise), it follows that once an event happened, it could not be otherwise from how it happened.

There is a book in the box, tomorrow it will rain, you'll pour some water in the glass... what's common between these facts is just your ignorance of a certain portion of space-time. It's raining, you pour the water - the fact that it didn't happen otherwise proves that these were the only possibilities (at least in this universe or hyper-time).

What happened is therefore necessary, for the simple fact that it's impossible that an event that took place in one way took place in another. After all, you know for sure that something will happen tomorrow, this is necessary - but you don't know what.

Note: this question is not strictly related to determinism. Causal determinism says that event X is determined completely by previously existing causes. In this case, X can happen even undeterministically. X would be unpredictable, but even before it happens the fact that X happens is necessary, although you may not know what X is. You know for sure that something (X) will happen tomorrow, but you don't know what. For SEP determinism is: "The world is governed by (or is under the sway of) determinism if and only if, given a specified way things are at a time t, the way things go thereafter is fixed [as a matter of natural law]". The brackets are mine, and are the part that I don't need in the case above.

I suggest a distinction. If X happened - you poured a glass of water on December 13 2018 at 10.57 hrs - the truth that it happened is a necessary truth in the sense that it is impossible for that historical truth to be false now or at any future time.

However, to introduce the distinction, it does not follow that your pouring that glass of water was a necessary event at that time. For all I know we may live in a world of total contingency where anything can and does happen undeterministically. You might even have had free will in a deterministic universe such that you had a choice totally unconditioned by your situation for action and the laws of nature (if there are any).

In such scenarios your action was not necessitated but, once it had been done, it was necessarily true, and in that sense is a necessary truth, that you had done that action.

Note

'Necessity' is not a notion I generally use but I suppress my reservations in the interests of the question.

Suppose that the universe is not deterministic (indeed, this is required for anything to not be necessary given the initial state of the universe). This can take whatever form desired - chance, free will, or whatever other position one can take.

Then an event A which happens at time B is necessary at time C if and only if, in all potential futures of C, the statement "A occurred at time B" is true. When we say that an event is necessary, we are frequently speaking in relation to the beginning of the universe, but other referents are possible.

Directly relevant to your example: you pouring water in the glass is necessary only if there was no possible world in which you did not do so, given (perhaps) the state of the universe that morning.

There is a second definition of necessary, which is related and similar. Something can be said to be necessary if there are no possible worlds in which it does not happen. For example, it is not necessary that there be something rather than nothing, or that entropy exist. It is necessary that logic holds, because it doesn't depend on anything contingent on any particular possible world.

This is a clever idea, but I have never managed to articulate it well. It is encouraging to see someone else express it though.

I think the words "numerically identical" belong somewhere in the explanation. Something along the lines of "A thing that occurs (or will occur) in a certain way must occur in that particular way in all numerically identical cases. If it does not occur or occurs differently then something has changed and the case is not numerically identical". Something like that anyway. Its a bizarre delightfully circular and bizarre but that does not mean its not true. Maybe only circular things can be really true, truth has to start somewhere.