The last two posts introduced and evaluated the Aristotelian View (AV) which grounds possibilities in the causal powers of substances. We discovered that a modal theory is preferable to other theories (ceteris paribus) insofar as it provides truth-makers that are epistemically accessible to us, and that the AV succeeds on this point because powers are often directly or indirectly known by us through our interactions with other substances. Moreover, we found that if causation is a bedrock concept for explaining other very general phenomena, then it can probably illuminate the nature of modality as well.
However, a strong objection to the AV is its difficulty accounting for possible worlds that are radically different from our own – sometimes referred to as “alien possibilities” – because the only resources at its disposal are the powers of finite substances in this universe. This is where Leibnizian Theism (LT) supplements what is lacking in the AV.
LT takes the notion of alien possibilities seriously. It recognizes that there might have been universes composed of no physical objects, governed by radically different laws, or populated by exotic entities like flying horses or replicas of ourselves that lack consciousness. LT solves the problem of alien possibilities by grounding all possibilities – not just alien ones! – in an omnipotent Being who has the power to chose whether to bring them about or not. Nothing is possible unless this Being can create or sustain it in existence. [In the paragraphs to follow, I will refer to this Being using the term “God,” even though it falls somewhat short of the divine concept endorsed by classical theism.]
Three important consequences follow from the claim that God is the ground of all possibilities. First, God is a necessary being who exists in every possible world. This means that for any possibility there is, God is in it, since otherwise he would not be the ultimate basis for it.
Second, God’s existence is not (strictly speaking) possible. Why? Here is the reason: if all possibilities are grounded in powers, and if God’s existence is possible, then that possibility is either grounded in (1) his own powers or (2) in those of another. But (1) is incoherent because it requires that God be causally prior to his own existence. And (2) is contradictory because (by definition) God grounds all possibilities. Therefore, God’s existence is not possible. As above, it is more accurate to say that God exists in every possibility by virtue of his being the basis for it.
Third, God is an independent being. If his existence were dependent, the possibility of his existence would be grounded in something other than himself, and we’ve already seen that this is contradictory. Therefore, God must exist by the necessity of his own nature as an uncaused, independent being.
According to Plato, abstract entities such as propositions exist necessarily and independently in a heavenly realm beyond space and time. But defenders of LT have traditionally departed from Plato’s view by seeing propositions as ideas in the mind of God.
One reason for their misgivings about Platonism stems from a discussion of intentionality. Intentionality is the characteristic of being of or about things. For example, maps are about landscapes, family portraits are about people, and newspapers are about events. But maps, photos, and newspapers are only about things because minds give them meaning; without minds they would be meaningless scratches of pigment on paper – nothing more. A similar point can be made about objects in the internal world of thoughts, concepts, and images. They are about things because minds give them meaning. What do these observations suggest? They suggest that intentional objects in general have the feature of being mind-dependent. But since propositions are also intentional objects, it follows (contra Plato) that their intentionality depends on mind.
But what kind of mind do propositions depend on? Leibniz agreed with Plato that propositions exist necessarily, apart from the human minds that contemplate them. If this is correct, then only the mind of God can account for their intentionality because only a necessary being could have them in his mind in every possible world. For this reason, Leibniz believed that propositions were concepts or “thinkings” in the divine intellect. [Note: if possible worlds are sets of propositions, then possible worlds are divine ideas as well]
Categories of Modality
In contemporary literature on metaphysics, it is commonplace to distinguish various categories of modality. Three of the most important kinds are known as “strictly logical,” “broadly logical,” and “natural.” Something is strictly logically impossible if it entails a contradiction. In this sense, we understand that the existence of a married bachelor is impossible because the very concept of a bachelor entails that one is unmarried; a married bachelor is a contradiction in terms because of the way we define concepts. But how should we understand the broadly logical and natural categories of modality?
LT defines these two remaining categories of modality by placing or removing restrictions on whose causal powers have a say in defining them. When restrictions are placed on whose powers can count, a category is narrowed. When restrictions are removed so that all powers can count, a category is broadened. For instance, consider broadly logical modality. On Alexander Pruss’ version of LT, “broadly logical modality [just is] causal modality [without any] restriction on whose causal powers count.” In this sense, something is possible if it is causable by anything that actually exists – whether by me, God, or some other substance. There is no restriction on whose powers have a say in defining what it possible. No doubt, the fact that an omnipotent God is allowed to play a defining role guarantees that this category will be the broadest of them all!
What about natural modality? Whose powers have a say in defining that category? Again, Pruss (2013) writes that “natural modality [just is] causal modality but with the restriction that we only consider the causal powers of natural agents.” In this sense, something is possible if it is causable by anything that is governed by the laws of nature – whether by galaxies, planets, molecules, atoms, or electrons. As long as something is causable by natural entities, it is possible in the natural sense. By implication, this category prohibits non-natural agents like God from playing a defining role.
Next week we will explore some of the benefits and drawbacks of Leibnizian Theism as an account of possibility.
 For example, Leibnizian theism is distinct from classical theism because the former conception can lack attributes like moral goodness, whereas the latter cannot.
 Leibniz’s idea that God is the basis for all possibility is consistent with the idea that other beings which depend on him can ground possibilities as well. Why? Because these other beings only exist and have capacities if God allows this to be so. For example, my ability to jump grounds the possibility of my jumping, but on LT, God was causally involved in the chain of events leading up to my existence and, therefore, to my having that ability.
 It is also worth noting that Leibniz’s view of propositions (and of abstract objects in general) is more parsimonious than Platonism. Instead of postulating an infinite number of independently existing propositions, Leibniz postulates just one necessarily existing Mind who inevitably generates these entities as part of its mental life.
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