The Modal Disunity of Metaphysical Truths

Abstract Systematicity has good claim to being a theoretical virtue. One way of cashing out this notion of systematicity is through the degree of unity of a theory. For instance, some such principle as the following might be plausible: for candidate theories T1 and T2, if the statements of T1 are more unified than those of T2, T1 is ceteris paribus to be preferred to T2. One regard in which a theory might be more unified than another is if its claims all have the same modal status. It seems to have an even greater degree of unity if the claims of the theory are characteristic of that modality (in a sense to be explained) and if the modality is a highly natural one. For instance, it is plausible that all the laws of perfected physical science are physically necessary, that these laws are characteristic of the modality of physical necessity, and that physical necessity is at least a reasonably natural modality. Jonathan Schaffer has argued that something similar is the case for metaphysics: the metaphysical truths are metaphysical laws and are all metaphysically necessary (Schaffer 2010). Furthermore, the truths of metaphysics are characteristic of metaphysical modality and metaphysical modality is highly natural. Against this, I argue that not all the truths of metaphysics are metaphysically necessary—at least on what I claim is the most natural conception of metaphysical necessity. In particular, I argue that we should conceive of metaphysical modality as the broadest alethic modality, and that this modality is subject to a unified logical treatment. The modality itself is thus highly systematic, even if the truths of metaphysics turn out to be disunified.