between impressions of sensation (found in sense experience) and impressions of reflection (found mainly in emotional experience), only to set aside any further discussion for Book 2's treatment of the passions. Returning to ideas, Hume finds two key differences between ideas of the memory and ideas of the imagination: the former are more forceful than the latter, and whereas the memory preserves the "order and position" of the original impressions, the imagination is free to separate and rearrange all simple ideas into new complex ideas. But despite this freedom, the imagination still tends to follow general psychological principles as it moves from one idea to another: this is the "association of ideas". Here Hume finds three "natural relations" guiding the imagination: resemblance, contiguity, and causation. But the imagination remains free to compare ideas along any of seven "philosophical relations": resemblance, identity, space/time, quantity/number, quality/degree, contrariety, and causation. Hume finishes this discussion of complex ideas with a sceptical account of our ideas of substances and modes: though both are nothing more than collections of simple ideas associated together by the imagination, the idea of a substance also involves attributing either a fabricated "unknown something, in which [the particular qualities] are supposed to inhere" or else some relations of contiguity or causation binding the qualities together and fitting them to receive new qualities should any be discovered. Hume finishes Part 1 by arguing (following Berkeley) that so-called "abstract ideas" are in fact only particular ideas used in a general way. First, he makes a three-point case against indeterminate ideas of quantity or quality, insisting on the impossibility of differentiating or distinguishing a line's length from the line itself, the ultimate derivation of all ideas from fully determinate impressions, and the impossibility of indeterminate objects in reality and hence in idea as well. Second, he gives a positive account of how abstract thought actually works: once we are accustomed to use the same term for a number of resembling items, hearing this general term will call up some particular idea and activate the associated custom, which disposes the imagination to call up any resembling particular ideas as needed. Thus the general term "triangle" both calls up an idea of some particular triangle and activates the custom disposing the imagination to call up any other ideas of particular triangles. Finally, Hume uses this account to explain so-called "distinctions of reason" (e.g., distinguishing the motion