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 (IH