  Physical objects are spatio-temporal, and bear spatio-temporal and causal relations to each other. Mental states seem to have causal powers, but they also possess the mysterious property of  intentionality – being about other things – including things  like Zeus and the square root of minus one, which do not exist. No mere physical thing could be said to be, in a literal sense, ‘about’ something else. The nature of the mental is both queer and elusive. In Ryle’s deliberately abusive phrase, the mind, as the dualist conceives of it, is a ‘ghost in a machine’. Ghosts are mysterious and unintelligible: machines are composed of identifiable parts and work on intelligible principles. But this contrast holds only if we stick to a Newtonian and common-sense view of the material. Think instead of energy and force-fields in a space-time that possesses none of the properties that our senses seem to reveal: on this conception, we seem to be able to attribute to matter nothing beyond an abstruse mathematical structure. Whilst the material world, because of its mathematicalisation, forms a tighter abstract system than mind, the sensible properties that figure as the objects of mental states constitute the only intelligible content for any concrete picture of the world that we can devise. Perhaps the world within the experiencing mind is, once one considers it properly, no  more – or even less – queer than the world outside it.