In lieu of an abstract, here is a brief excerpt of the content:Hume's Demarcation Project John Losee Demarcation, Ideas and Impressions David Hume sought to exclude certain concepts from the domain of empirically significant discourse. He was critical of talk about "substances" that bear qualities, "forces" that cause motions, "powers" that produce effects, "necessary connections" that determine sequences of events, "extension without matter" and "time independent of succession or change in any real existence."1 Hume proposed a demarcation ofideas, a demarcation (...) intended to exclude these notions. The demarcation project, if successful, would exclude Locke's notion of "substance" as well as Newton's notions of "Absolute Space" and "Absolute Time." Some interpreters have taken the demarcation project to be the centrepiece of Hume's philosophy. Others have seen it to be an incidental aspect of Hume's search for an empirical theory of human nature. I believe that David Pears was correct to insist that an acceptable account ofHume's philosophy include both the "destructive emphasis" and the "constructive emphasis."2 Hume's demarcation project is based on a presumed relation, or relations, between ideas and impressions. Hume made the following claims about "simple ideas," those ideas that "admit of no distinction nor separation" (T 2): 1)every simple idea in its first appearance is derived from some impression (Derivability Principle); 2)every simple idea in its first appearance copies some impression (Copy Principle); 3)every simple idea represents some impression (Representation Principle); 4)simple ideas and impressions resemble one another; 5)simple ideas correspond to impressions; 6)every simple idea is borroVd from some impression; Volume XVII Number 2 51 JOHN LOSEE 7)for every simple idea there is some temporally antecedent impression; and 8)impressions cause simple ideas. Primafacie, these claims are empirical claims about the genesis of our ideas. But within Hume's demarcation project these claims also function as criteria for membership in the class of simple ideas. Those ideas that satisfy the criteria constitute raw material from which complex ideas are formulated. Complex ideas are constructed from simple ideas. However, they are not copies of simple ideas. Aparticular complex idea is empirically significant provided that, 1) it is derivable from simple ideas; and 2) each of those simple ideas is derivable from impressions. The Case of the Missing Shade ofBlue Tensions arise within the demarcation project in part because Hume employs nonequivalent criteria—Derivability Principle, Copy Principle, Representation Principle—as if they were equivalent. The tension is acute in the case of the missing shade ofblue. The "missing shade" is an idea formulated by a person with extensive experience of colours, but no prior exposure to the shade in question. If this idea is a simple idea, then it is false that every simple idea copies some antecedent impression. Why did Hume cite—in both Treatise and Inquiry—this apparently destructive exception? Bernard Rollin has suggested that Hume may have cited the missing-shade-of-blue example as a rhetorical device toprotecthis dichotomous divisionofpropositionsinto those that are analytic a priori and those that are synthetic a posteriori (including the Derivabihty Principle).11 John O. Nelson agreed that Hume is concerned to emphasize the contingent, empirical status of the Derivabihty Principle.12 Nelson suggested that Hume sought tomake two points. The first pointis that an exception to the Derivability Principle is empirically possible. The second point is that thought-experiments like the missing-shade example are inadequate to establish conclusions about matters offact. To entertain such thought-experiments is to lapse into hypothetical speculation, a violation ofthe "experimental method ofreasoning."13 Perhaps it was for this reason that Hume dismissed the missing-shade case as unimportant. Hume urged that this case is "so particular and singular" thatit provides nogoodreason to abandon the Copy Principle (T 6). 52Hume Studies HUME'S DEMARCATION PROJECT D. M. Johnson has developed a hypothesis to explain why Hume believed that this exceptional case is not damaging to his demarcation project. According to Johnson, Hume may have believed that for a person to reconstruct a shade he never saw before, he must have developed an appropriately detailed Tiabitual spectrum'—i.e., an ordered set oftendencies and expectations concerning colours, built up as a result oflong experience with thingshavingthese colours—whichhe then employs... (shrink)