What is Hume’s Dictum, and Why Believe It

Hume famously said “There is no object, which implies the existence of any other if we consider these objects in themselves”.1 A typical, general, contemporary version of Hume’s Dictum reads: (HD) there are no metaphysically necessary connections between distinct, intrinsically typed, entities. HD plays a key role in many metaphysical debates. Beyond Hume’s original application to the case of causal connections, HD serves, for example, as ultimate reason to accept some combinatorial account of modality (Lewis 1986; Armstrong 1989), and to reject states of affairs (Lewis 1992) and necessitarian accounts of properties and laws (Armstrong 1983). Especially in its combinatorialist guise HD crops up as a crucial premise (see van Cleve 1990 and Kirk 1996 in defense of supervenience-based formulations of physicalism, and Paull and Sider 1992, Bennett 2004, and Moyer forthcoming on whether certain supervenience relations are equiva-lent). Reflecting this influence, HD’s bearing on various positions is now a philosophical topic in its own right (see Hawthorne et al. 2006 on whether HD motivates “4-dimensionalism ” about persons, and Cameron 2006 on whether HD is compatible with tropes ’ being non-transferable). One should ask of such an influential thesis: why believe it? Interestingly, arguments for H