Disclaimer: the following remarks are highly exploratory and are not to be read as expressing anything final.
1. What is the status of the claim with which Lewis begins On the Plurality of Worlds? (That there are other ways the world could have been other than the way things are.) We have a tendency to hear it in the wrong key. Imagine someone who had lost a loved one saying 'Things really could have been different'. (There seems something self-deluding in this, although it is no ordinary self-delusion. You don't fix it with negation.)
2. Ayer, Carnap and others wanted to say: the necessities fall out of our linguistic/conceptual scheme. Codify 'the rules of correct English' (cf. Language, Truth and Logic) and you've codified the necessities (or a base from which they may be logically derived). But the Kripkean necessary a posteriori, semantic externalism etc. make it clear that one needs a lot more fine structure than 'rules of correct English', and that some structures are in some sense not correct, in a way which has nothing to do with internal incoherence, but rather to do with inadequacy to an external referent.
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3. 'I went to X, but could have gone to Y instead.'
How might such a proposition be used? A retrospective epistemic use. Or to point to various facts - e.g. that Y wasn't too far away, or that I was under no compulsion to go to X, or that I was seriously considering going to Y, etc. But what if I say that none of that is quite to the point? 'Sure, those things might be true, but over and above that, I'm saying that I really could have gone to Y. The world could have gone differently here, and my going to X was just how it happened to turn out.' What could this mean? (We can imagine someone being bothered, preoccupied, by the thought that things really could have gone differently.)
4. Consider these propositions:
(1) There are things which could have gone otherwise.
(2) There are contingencies.
When a philosopher asserts (1) or words to that effect, typically the reaction they want is 'of course!'. Now, this would happen sometimes, but more common among non-philosophers might be perplexity - 'what do you mean?' - and skepticism - 'but how do you know?'. (I have some experience of this.)
5. Both in and out of serious philosophy, there is a tendency to take (1) and (2) in what I would call the wrong way. Roughly: as though they described something extremely fundamental and general about reality. And yet that is not an entirely wrong-headed thought, for it is immensely important that we talk and think about non-actual scenarios.
6. It is as though, when someone pushes a statement of counterfactual possibility in a certain way, we take them to be making a very strong claim, quite beyond our ken.
7. When one hears (1) and (2) in an "inflated" way - whatever that comes to - necessitarianism can seem like the antidote: something wrong has been said, and so it should be denied. But then the denial turns out to be just as troublesome, if not more! (Now it is trivially false, instead of trivially true - like Idealism in relation to Realism.)
8. The disease is not fully containable in any one proposition.
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9. It can be very important to realize that some propositions are necessary - 'One cannot feel another's pain', 'The opium worked (if it worked at all) by means of a dormitative power'. Likewise, contingent - 'There are two sexes', 'We have one body each', 'We use the decimal system to count'. It can be very instructive to get clear about the "modal status" of these things. And of course this does not mean leaving the view that they are contingent (or necessary) and coming over to the other side, but rather: noticing something, which one might have been quite blind to. Think of the conceptual development of children - one cannot truly say that they go through a phase of believing that everyone knows the same as them, but we can see what this is getting at. In the place where we have a fundamental distinction or concept of great importance in adult life, they do not have anything to speak of.
10. So: it can be important to realize the modal status of certain propositions. But - and this is crucial - it does not at all follow from this that it is important, desirable or possible to draw a line which clearly divides all propositions according to modal status.
11. What is the modal status of 'Our conceptual scheme has moving parts'? I.e. how could anything else be a conceptual scheme? (Mind of God.) And yet the proposition easily acquires the flavour of a general fact of nature. Why? Well, lots of important facts lie immediately underneath.
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12. The notion of the mind of God. It can look as though everything is an approach to this ideal, and yet we have absolutely no grip on the ideal. We (i.e. myself and most active philosophers) do not have any positive belief in the mind of God in the old sense, and all the minds we do believe in are completely un-Godly.
14. So now it looks like, in a sense, the ideal conceptual scheme has no moving parts - no need to dance further with experience or reason. And yet we will always need a conceptual scheme with moving parts, and can really make no sense of anything else. Thus how could the mind of God be our ideal?!
15. From this perspective, one could imagine pantheism as a kind of scholastic solution to our present problem. (For it would block the assimilation of our mind with God's.) ('A full, non-redundant model of an object must be a duplicate of the object, or the object itself.')
16. We say things could have been otherwise - I could have walked somewhere else today. But then someone says 'but could that really have happened?' and it's like the stakes are raised somehow. But isn't this a shift to a more specialized modality?
17. "What makes you think things could have really been different, and that this isn't just a function of our ignorance?" - The first thing to say is there's no simple answer here: one can't just point to evidence. The question is symptomatic of something.
18. One pernicious kind of confusion when talking about metaphysical necessity - the modality identified, and separated from other concepts, by Kripke - is to think of it in a manner which would be appropriate for some kind of deep and remote form of retrospective epistemic possibility. This is illustrated by the notion of the Creation as an event. God did something, and it may have been reasonable to expect this to go various ways. It is important for our concerns here that such an idea is natural to some degree.
19. It is possible to feel that any kind of conceptual view of necessity, even if we grant everything to the concepts including external adequacy, somehow makes modality flimsy, shadowy - somehow still overlooks reality in some way. "The way things really could have been, concepts aside." (The notion of an inconceivable object. If such a thing is impossible, how is that not just a piece of luck? Thus it can seem that the conceptualist must rely on some kind of pre-established harmony between possible concepts and possible realities, which would of course be highly suspicious.)
20. There is a strong philosophical tendency to think of knowledge of how things must be as penetrating deeper into the world than mere contingent knowledge. As though we saw how a machine behaved, but then looked into its workings and saw that it had to behave that way. Or, we form a hypothesis about the workings based on, and in order to make sense of, the behaviour. (This gets us into difficulties, but it would be stupid to call it incorrect - or correct for that matter.)