 Such is an expression of the “error … deeper and more fundamental than any other” with which Mackintosh charged Hartley: that of overlooking ontological dualism, “the primordial and perpetual distinction between the being that thinks and the thing which is thought of” (1836, 245). Descartes maintained this distinction by positing a substance dualism of mind and body (and by treating animals as “mere” mechanisms, without consciousness, and humans as rational, sentient beings), and such as Hume and later James Mill respected it (or at least avoided dealing with it) by writing about the contents of conscious minds while keeping well clear of neurophysiology. But Hartley ignores the distinction between “thinking” and “thought of”; he affirms that neurological processes generate consciousness.