The case for modal realism can be motivated in a few ways. It often begins with contemplating the everyday modal propositions we make about the world. We seem to know that things might have been different, after all, and there are different ways the world could be in the future. Some dinosaurs might have survived the meteor catastrophe. I might have tried to avoid traffic by leaving earlier this morning. It’s possible that I might stop writing this post now, and finish it later. By virtue of what can these propositions be considered true or false? Being of realist persuasion, I think there must be something in the reality outside our minds to provide truthmakers for these propositions (my prior posts on modality and modal realism can be found here).
Modal realism also seems linked to realism about causality, when the causal connection is seen as a counterfactual dependence. I might not have checked my e-mail: if I hadn’t, I would not have seen your message.
In a deflationary metaphysics where there exists one world subject to deterministic laws, all connections are necessary. Possibility and contingency would only be illusions. They would only exist in our minds. (Likewise it seems to me that the directional flow of time and causality would have no parallel in the world; necessary connections are symmetric.)
But given a naturalistic worldview, our minds arise from the same stuff as the rest of the world. Is it plausible that a world which lacks real possibility would give rise to creatures for whom the notion is indispensable?
Of course, we know strict determinism is false, given the real indeterminism present in quantum mechanics. I have argued elsewhere for an interpretation which sees the quantum states as incorporating real (although not concrete) possibilities, while measurements are concrete actualization events.
Leaving aside for now the tough problem of describing the modal space in any detail (see note below), I am intrigued by the notion that the modal propositions and modal reasoning we employ are grounded in a modal reality.
The thesis of modal rationalism, as explored by David Chalmers in section 10 of this paper, is the idea that our notion of what is conceivable (logically possible) does indeed match what is metaphysically possible. He examines and argues against proposals that these need be distinct modal spaces.
If I couple modal rationalism with a modal realism which posits that metaphysical possibilities really exist (outside the cranium), this creates what I think is a fascinating picture. We, along with all natural phenomena, are continually actualized from a space of possibilities: our roots in this space form the basis of our evolved faculty for modal reasoning.
Note: For a great discussion of how to flesh out the space of metaphysical possibility, and related issues including modal rationalism, I recommend Richard Chappell’s recent draft paper. [UPDATE 25 January 2010: here's a link to a pdf of the final paper from Chappell's website.]