Kant's Revolutionary Theory of Modality by Uygar Abaci (review)

Uygar Abaci's Kant's Revolutionary Theory of Modality starts with a helpful and illuminating historical contextualization of Kant's theory of modality. It sets out the ontotheological debates that form the backdrop of Kant's pre-Critical modal theorizing. Abaci covers the proofs of the existence of God by Anselm and Descartes, as well as Leibniz and Wolff. The first two start from the idea of God as the ens perfectissimum and then try to establish the existence of God by arguing that existence is a perfection. The last two, by contrast, consider God to be the ens necessarium, such that God exists necessarily, provided the concept of God is not self-contradictory; they then attempt to establish the possibility of God by identifying the ens necessarium with the ens perfectissimum, which allows them to argue that this concept only contains positive predicates that cannot contradict each other.

The second part of Abaci's book covers Kant's pre-Critical modal theory, both his critique of the ontological argument in terms of existence not being a real predicate and his own attempt to establish the existence of God in "The Only Possible Ground of Proof for a Demonstration of the Existence of God" of 1763 (OPA). Kant is standardly taken to depart decisively from his logicist predecessors by introducing a conception of real modality in the OPA. Once real possibility is not merely a matter of conformity to the logical law of noncontradiction and, correspondingly, real necessity is not a matter of logical necessity, room opens up for Kant's argument that, even though God's existence is not logically necessary (given that existence is not a real predicate), it is nevertheless really necessary, since God is the ground of all possibility such that God's non-existence would cancel all possibility.

Abaci argues that Leibniz and Wolff already had a conception of real modality and that Kant's pre-Critical work on modality is, accordingly, merely revisionary and belongs to the same paradigm as that of his predecessors. He correctly points out that some rationalists, most notably Leibniz, accepted material conditions on possibility in the form of an actualist principle, according to which every possibility must be grounded in something actual. These material conditions, however, have no bearing on what possibility consists in—that is, on what it is to be possible—but only on which things are possible. In other words, they concern what material there is to which the conditions of possibility can apply. A genuine account of real modality would also include nonlogical formal conditions on what it is to be possible, something that neither Leibniz nor Wolff countenances. Attributing a conception of real modality to them is thus somewhat tenuous. Abaci's claim that Kant's OPA theory is broadly continuous with that of his predecessors can nevertheless be sustained. In particular, one might think that Kant himself does not have a genuine conception of real modality in the OPA and that this is one of the crucial innovations of his Critical modal theory. Abaci provides strong arguments against the standard interpretation that Kant recognizes metaphysical incompatibilities in the OPA. He nicely illustrates how real opposition, which involves opposed grounds that cancel each other's consequences, implies a logical contradiction in the case of the ens realissimum, insofar as real opposition results in a lack or defect that is logically incompatible with having all perfections to a maximal degree, thereby illustrating that countenancing real opposition need not imply countenancing real modality.

Whereas Abaci considers the pre-Critical modal theory to be merely revisionary, he finds its Critical counterpart revolutionary (though he recognizes that the core ideas are already, at least implicitly, present in the OPA and that their systematic development naturally leads to the Critical modal theory—in fact, Abaci nicely sets out how theorizing about real modality plays an important instrumental role in prompting the Critical turn). The third part of the book comprehensively covers the core...