  Reid offers two criticisms of the ancient theory, as he understands it. First, the theory falls afoul of one of Reid’s own methodological strictures, namely, that a theory must adhere to Newton’s regulae philosophandi, or rules of philosophizing (Inquiry, 12). The first rule is to posit no merely theoretical causes and in Reid’s view the second rule forbids positing causes insufficient to explain the phenomenon in question. According to Reid, there is no observational evidence of the existence of impressions on the brain—they are merely theoretical entities (Essays, 281). Furthermore, even if we granted the otherwise theoretical existence of impressions, such entities would not be sufficient to explain memory. We might establish a correlation between impressions and memories, but it would remain at best just that: a correlation, not a causal explanation. Having learned Hume’s lessons about causation, Reid denies any necessary connections between impressions and memories sufficient to regard the former as a cause of the latter. Reid also considers whether resemblance could ground such a causal explanation, but, having learned Berkeley’s lessons about resemblance, he denies that any mental states can resemble material states such as impressions on the brain. Reid’s second criticism is that even if we were to grant that impressions remain after the objects that impressed upon the senses are gone, this would entail that we should continue to perceive objects rather than remember them, since on the ancient theory, impressions are the immediate causes and objects of perception (Essays, 282).