 Do the different sorts of mental state form natural kinds? We certainly think that our minds make a difference to what happens in the physical world and we think that we act because we have certain beliefs, desires, hopes, fears etc. The distinctive explanatory roles performed by these different states in folk psychology certainly suggest that these beliefs, desires, hopes and fears constitute distinct mental natural kinds. Note that in considering natural kinds in chemistry, biology, and physics we have thought of kinds principally as kinds of thing (kinds of stuff, organism, particle, etc.) whereas we are now considering the possibility of certain kinds of state. (Ellis (2001, 2002) is clear that he does not limit natural kinds to kinds of thing, but includes kinds of state and process also.) A Cartesian dualist holds that mental states are distinct from any physical state of the subject, as they are states of an immaterial thinking substance. The problem for dualism however is in finding the ground of the difference between fearing, believing, hoping etc. i.e. what difference is there to the immaterial substance when a subject is in different mental states? In principle this problem could be avoided by the dualist if mental kinds are regarded as immaterial substances (Shoemaker 2003).