  Whether one believes that the mind is a substance or just a bundle of properties, the same challenge arises, which is to explain the nature of the unity of the immaterial mind. For the Cartesian, that means explaining how he understands the notion of immaterial substance. For the Humean, the issue is to explain the nature of the relationship between the different elements in the bundle that binds them into one thing. Neither tradition has been notably successful in this latter task: indeed, Hume, in the appendix to the Treatise, declared himself wholly mystified by the problem, rejecting his own initial solution (though quite why is not clear from the text).