Laws and constrained kinds: a lesson from motor neuroscience
In this paper, I want to explore the question of whether or not there are laws in psychology. Jaegwon Kim has argued (Supervenience and mind. MIT press, Cambridge; 1993; Mind in a physical world. MIT press, Cambridge 1998) that there are no laws in psychology that contain reference to multiply realized kinds, because statements about such kinds fail to be projectible. After reviewing Kim’s argument for this claim, I show how his conclusion hinges on a hidden assumption: that a kind can only feature in a projectible statement if it is defined by an internal physical property. This assumption, however, is false: constrained kinds can feature in projectible statements, and yet they are not defined by any set of internal physical properties. I suggest that many mental terms actually refer to constrained kinds, and give an example from motor neuroscience of a constrained kind that is multiply realizable and “projectible”: the intention to move voluntarily in a specific direction.