reason). But since the truth "must lie very deep and abstruse" where "the greatest geniuses" have not found it, careful reasoning is still needed. All sciences, Hume continues, ultimately depend on "the science of man": knowledge of "the extent and force of human understanding,... the nature of the ideas we employ, and... the operations we perform in our reasonings" is needed to make real intellectual progress. So Hume hopes "to explain the principles of human nature", thereby "propos[ing] a compleat system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security". But an a priori psychology would be hopeless: the science of man must be pursued by the experimental methods of the natural sciences. This means we must rest content with well-confirmed empirical generalisations, forever ignorant of "the ultimate original qualities of human nature". And in the absence of controlled experiments, we are left to "glean up our experiments in this science from a cautious observation of human life, and take them as they appear in the common course of the world, by men's behaviour in company, in affairs, and in their pleasures". Book 1: Of the Understanding Part 1: Of ideas, their origin, composition, connexion, abstraction, etc. Hume begins by arguing that each simple idea is derived from a simple impression so that all our ideas are ultimately derived from experience: thus Hume accepts concept empiricism and rejects the purely intellectual and innate ideas found in rationalist philosophy. Hume's doctrine draws on two important distinctions: between impressions (the forceful perceptions found in experience, "all our sensations, passions and emotions") and ideas (the faint perceptions found in "thinking and reasoning"), and between complex perceptions (which can be distinguished into simpler parts) and simple perceptions (which cannot). Our complex ideas, he acknowledges, may not directly correspond to anything in experience (e.g., we can form the complex idea of a heavenly city). But each simple idea (e.g., of the colour red) directly corresponds to a simple impression resembling it—and this regular correspondence suggests that the two are causally connected. Since the simple impressions come before the simple ideas, and since those without functioning senses (e.g., blindness) end up lacking the corresponding ideas, Hume concludes that simple ideas must be derived from simple impressions. Notoriously, Hume considers and dismisses the 'missing shade of blue' counterexample. Briefly examining impressions, Hume then distinguishes