Things don't get a lot better if we seek help from Hume's comrade in empiricism John Locke.  In the first edition of An Essay Concerning Human Understanding, Locke describes memory as the “storehouse” of ideas, but he never offers an account of what distinguishes a memory idea from an idea of imagination.  And while the storehouse metaphor remains in the second edition of the Essay, Locke nevertheless backs away from it dramatically. He argues that because ideas don't exist unperceived there is no significant sense in which the ideas of memory continue to exist when we are not thinking about them.  Rather the mind has the power to “revive” ideas that we are said to remember.  Be this as it may, although Locke is clearly a proponent of RTM, he does nothing to help us answer the question of what it is that distinguishes memory images from those of other kinds. We'll look at Locke's account a little more closely when we discuss Reid's criticisms of the RTM.