In two previous posts, I highlighted some important facets of modal epistemology (henceforth, M.E.), the discipline that explores the justification of claims about what is possible, impossible, and necessary. In those posts I briefly touched upon some objections to modal knowledge from the empiricist tradition of David Hume and from the Analytic Philosophy of Language. I tentatively concluded that the justification for modal claims should be understood as contextual, fallible, defeasible, and amenable to multiple sources of support.
The philosophy of modality is important because claims about possibility, impossibility, and necessity show up in so many disciplines, including ethics, logic, physics, theology, and metaphysics. Ethicists often say that persons are responsible for their actions only if they could have acted otherwise; meta-ethical theorists of a realist persuasion have argued that moral truths such as ‘It is wrong to torture babies for fun’ (or at least the values those truths are based upon) are broadly logically necessary, even if their denials do not (strictly speaking) entail contradictions; logicians assert that it is impossible for the premises of a valid deductive argument to be true, but its conclusion false; physicists speculate about what it would be like for universes to be governed by different laws of nature; theologians wonder if God could have created worlds that contain less human and animal suffering; and metaphysicians ask questions like ‘must anything at all exist?’ and ‘is it possible for water not to be H2O?’ In each of the above examples, modal notions are presupposed and modal claims are subject to rational argumentation and reflection.
In this week’s post, I want to shift gears away from questions of epistemology towards issues of ontology. The discipline of modal ontology (henceforth, M.O.) is distinct from M.E. because it is not directly concerned with how we know modal propositions to be true; rather it explores that in virtue of which modal propositions are true. The rationale behind M.O. is that some kind of grounding is needed to account for why statements are true, particularly modal statements. Grounding is normally understood in terms of the truth-makers of statements. For example, suppose it is true that ‘Hitler might not have existed’. If so, what makes it true that Hitler might not have been born? One way of grounding that fact about Hitler is to say that his parents had the capacity to refrain from copulating. Their capacities make it true that ‘Hitler might not have existed’. But are there other ways of grounding that fact? More generally, are there other theories of how to ground modal facts?
In Part One of his book Actuality, Possibility, and Worlds (2011), Alexander Pruss examines eight theories of the nature of possibility by exploring how well they account for possible worlds and the grounds for modal statements. These theories are (1) the Necessitarian view, (2) Extreme Modal Realism, (3) the Linguistic view, (4) the Propositional Primitive Modality view, (5) the Aristotelian view, (6) Leibnizian Theism, (7) Modal Irrealism, and (8) Conventionalism. In the weeks to follow, I will briefly describe and evaluate each theory according to its explanatory benefits and drawbacks.
 Perhaps some true statements can do without truth-makers, but surely a theory that supplies truth-makers is preferable to a theory that doesn’t – all other things being equal. As Alexander Pruss notes, “it is in general preferable in a philosophical theory of some proposition p that one be able to say more about the truthmaker of p than that it is its being the case that p. Being able to say more about this truthmaker is itself a reason in favor that theory. Thus, even if we do not want to insist that always more can be said, we will ceteris paribus prefer a theory that says more.” See his Actuality, Possibility, and Worlds. (New York, NY: Continuum, 2011, p.19). Note: with respect to a negative proposition, the absence of a falsemaker (not the presence of a truth-maker) would be adequate to ground its truth.
 For purposes of expediency, I sometimes use different names than the ones Pruss uses for the modal theories outlined in his book.