from the secondary qualities. Thus if we follow solid reason and exclude the latter, we will be forced to contradict our own senses by excluding the former as well, thereby denying the entire external world. Hume then examines "the nature of the mind", starting with the materialist-dualist debate over the substance of the mind. He rejects the whole question as "unintelligible", for we have no impression (and therefore no idea) of any substance, and defining "substance" as "something which may exist by itself" does not help (each of our perceptions, Hume argues, would then count as a distinct substance). Turning to the question of the "local conjunction" of mind and matter, he considers and endorses the anti-materialist argument which asks how unextended thoughts and feelings could possibly be conjoined at some location to an extended substance like a body. Hume then provides a psychological account of how we get taken in by such illusions (in his example, a fig and an olive are at opposite ends of a table, and we mistakenly suppose the sweet figgy taste to be in one location and the bitter olive taste to be in the other), arguing that unextended perceptions must somehow exist without having a location. But the contrary problem arises for dualists: how can extended perceptions (of extended objects) possibly be conjoined to a simple substance? Indeed, Hume waggishly adds, this is basically the same problem that theologians commonly press against Spinoza's naturalistic metaphysics: thus if the theologians manage to solve the problem of extended perceptions belonging to a simple substance, then they give "that famous atheist" Spinoza a solution to the problem of extended objects as modes of a simple substance. Finally, Hume examines causal relations, arguing on behalf of materialists that our observations of regular mind-body correlations are enough to show the causal dependence of the mind on the body, and that, since "we are never sensible of any connexion betwixt causes and effects" in general, our inability to detect any a priori connection between mind and body does nothing to show causal independence. Finally, Hume weighs in on the topic of personal identity. Notoriously, he claims that introspective experience reveals nothing like a self (i.e., a mental substance with identity and simplicity), but only an ever-changing bundle of particular perceptions. And so he gives a psychological account of why we believe in personal identity, arguing that "the identity, which