It's all about the deep questions.
Third Principle on the Structure of Possible Worlds: Real Modality Part 1
The whole intuition behind modality is that the world genuinely could have been otherwise. This thought, however, seems to be forgotten in the philosophical literature. To establish this point, we must first notice that there are two ways one can interpret modal claims.
The metaphysically light way: To say that X is metaphysically possible is just to say that X does not imply any logical contradiction. Some philosophers would want to add some other restrictions, so that X is possible is just to say that X lacks (or has) some feature M as well as not implying any logical contradictions.
The metaphysically heavy way: To say that X is metaphysically possible is to say that X genuinely could have been the case.
Being possible in this metaphysically heavy sense implies being possible in the metaphysically light sense. However, it does not follow, at least without further argument, that being possible in the metaphysically light sense implies being possible in the metaphysically heavy sense. Deflationists or necessitarians, for example, would agree that the set of non-actual X’s that are metaphysically possible in the light sense is non-empty, but they would say that the set of non-actual X’s that are metaphysically possible in the heavy sense is empty. In other words, it is not analytic that “X implies no logical contradiction means that X genuinely could have been the case. The right side is claiming something stronger that the left side.
The metaphysically light sense is philosophically uninteresting since it is often a trivial matter whether something implies a contradiction. The interesting philosophical question that originally motivated modality is the thought that reality really could have been different that the way it is.
Couched in these terms, much of the present philosophical debate seems wrong headed. What does the existence of a maximally consistent set of sentences that includes X have anything at all to do with the claim that X genuinely could have been the case? The existence of such a set would certainly show that X is possible in the metaphysically light sense, but it seems to have no bearing at all in the heavy sense. Also, what does the existence of proxies that half-exist (if that notion is even intelligible, to me it is not) have anything to do with the fact that it genuinely could have been the case that they full(?)-exist? Even on Modal Realism, there is nothing at all in reality that could have genuinely been different. Reality would just be all these spatiotemporally isolated worlds (if that notion is sensical, I will argue later that it isn’t).
It seems that every view on the market has absolutely nothing to do with genuine possibility!
What could be an account of genuine possibility? Is anything genuinely possible? I will argue that reality could not have been genuinely different. Then, I’ll draw some conclusions as to how we should modify our modal discourse in part 2
The Open Question Argument for Modality:
The argument is a nod to G. E. Moore’s Open Question Argument for Morality. He argues that since it is always “conceptually open” to ask whether something is really good given that it has such and such natural, physical properties, then the truth-maker for “X is good” cannot be natural, physical properties.
Hand wavy version: Given that reality in fact has some particular feature F, for any F, could another complete description of reality have been realized as opposed to the one that in fact is? This is conceptually open question
Non-hand wavy version: Consider the statement, it is possible that X exists, where X does not in fact exist. Reality is completely specified by all the things that exist and all the intrinsic and relational properties between those things. No object can make it the case that X is possible. In other words for any Y, Y not equal to X, the question “Given that Y exists, could X have existed?” is always an open question. In other words, any conditional in the form, “If Y exists, then X is possible” is not analytically true. Similar remarks can be made for the intrinsic properties of any actually existing thing: they don’t make it the case that some other thing possibly exists, because the intrinsic properties are “about” the thing in question rather than some other thing. Lastly, relational properties can only be held between things that exist, so those similarly cannot serve as truth-makers. Since modal claims need truth-makers in reality, and we have shown that no fact about reality is up to the job, reality couldn’t have been different.