Locke on Substance, Mode, and Personal Identity

In my dissertation I examine how John Locke’s conceptions of “substance” and “mode” inform his theory of personal identity. My goal is to get a better understanding of what Locke’s picture of persons looks like, and where Locke lies within the larger debate over personal identity. I start with the persistence conditions Locke gives for persons. In Book II, Ch. XXVII of An Essay Concerning Human Understanding, Locke famously claims that sameness of substance is neither necessary nor sufficient for the identity of a person over time. Many commentators have contended that there is a tension between this claim and Locke’s definition of “person.” They argue that the latter makes it look like persons are substances, but the former makes it look like this can’t be the case. This has caused some commentators to argue that Locke thinks persons are modes. This has caused others to claim that Locke thinks persons are substances, but Locke means something different by “substance” when he gives the persistence conditions for persons than when he deems an entity a substance. Although substance readings of Locke on persons were quite popular for some time, mode readings have gained considerable traction as of late. I argue that we must get a firm grasp on what Locke means by “substance” and “mode” to come to a conclusion on the matter. After giving a thorough treatment of Locke on substance and mode, I swim against the current tide in the secondary literature and argue that there is compelling evidence that Locke thinks persons are substances. This becomes clear if we examine Locke’s definition of “person” in light of what Locke says about substance, power and agency in other parts of the Essay. Moreover, I argue that when we place Locke’s claims about sameness of substance in their proper context and see what he means by them, it becomes clear that there is no tension between Locke’s definition of “person” and the persistence conditions Locke gives for persons. Most importantly, we don’t have to attribute to Locke a conception of “substance” he doesn’t have in order to get this result. This is not to say that I think Locke’s picture of persons is without problems. It’s just that a tension between Locke’s definition of “person” and the persistence conditions Locke gives for persons is not one of them.