I’m working on my first draft of Key Terms in Philosophy of Mind, a book under contract with Continuum Books. From time to time I’ll be posting draft entries on Brain Hammer, especially for controversial or especially difficult to arrive at definitions. Here’s “modal argument”:
modal argument, an argument for property dualism (see DUALISM, PROPERTY) in which key roles are played by the modal concepts of necessity and possibility or contingency. The basic gist of the argument involves arguing from premises concerning the necessity of identities (if x=y then necessarily x=y) and the contingency of any relation between mental and physical properties. Since, allegedly, for any physical property, it is possible for it to be instantiated without any mental property to thereby be instantiated, no physical property is identical to any physical property. According to a version of the modal argument formulated by Saul Kripke, although all identities, if true, are necessarily true, some identities, such as the identities found in natural science (like “water is identical to H2O”) seem contingent. According to Kripke, the appearance of contingency for such identities can be explained away in the following manner: what is contingently related to H2O is the watery appearance to us of H2O. While H2O is necessarily water, H2O is not necessarily water-appearing to us. So, any apparently contingent identity, that is, any apparently possible non-identity, is not really a non-identity if the appearance of contingency can be explained away in terms of a contingent relation between the appearance and the reality of a phenomenon. Contrapositively, if some apparent possible non-identity cannot be explained away in such a manner, then it is a real non-identity. Kripke offers that the apparent contingent relation between PAIN and neurophysiological events (“c-fibers firing”) does not admit of any such explaining away. Since, according to Kripke, anything that appears to the mind as a pain just is a pain: there is no distinction between the appearance of pain and the reality of pain. In versions of the modal argument due to David Chalmers, the contingency of mental-physical relations is supposed to follow from the conceivability of hypothetical scenarios such as the INVERTED SPECTRUM and the ZOMBIE.