  Reid's Sensory Deprivation Argument proceeds as follows. First, Hume's stated justification for the Copy Principle is inductive: he challenges people to find an idea that is not derived from a sensory impression, after he says that it appears all his ideas are copied from sensory impressions. But that, says Reid, is an exceedingly weak justification (EAP 1.4, 23). Besides, Hume's claim that the principle is “certain” is mistaken because the argument he sets out for the principle is inductive (IHM 5.7, 69–70, 75–6). After those ground clearing moves, Reid begins the argument proper with a thought experiment. Exhibiting affinities with Condillac's remarkable Treatise on Sensations (1754), Reid asks his readers to imagine a blind adult subject. Not only has he lost his sight, but he has “lost all the experience, and habits, and notions he had got by touch; [he lacks] the least conception of the existence, figure, dimensions, or extension, either of his own body, or of any other; but to have all his knowledge of external things to acquire anew, by means of sensation, and the power of reason, which we suppose to remain entire” (IHM 5.6, 65). Reid imagines introducing this blind experimental subject to a number of tactile sensations, one by one, from the most simple to the most involved: a single pin prick in a split second of time; a blunt object pushed against a small surface area of skin; the same object pushed against skin over a period of time “with a force gradually increased till it bruises him”; an object covering a large surface area of skin, applied over a period of time, and so on. Reid's emphasis on tactile sensations is important to understand: he uses this running example because ideas of primary qualities are the most important for gaining knowledge of the world and are the ideas essential for any science.