When mental operations share input and output mechanisms, it is necessary to resort to neuropsychological evidence to determine whether they belong to the same or different systems. That multiple memory systems exist is suggested by dissociation between memory performances by normal people, and proven by dissociations in memory performances induced by focal brain damage. The pathological forgetting of the amnesic syndrome is limited to memory for events ("episodic memory") and appears to be due to the patient's inability in escape from control of his present internal and external environment in order to recollect (consciously, by reexperience) a prior event. Effects of prior experience on subsequent performance that does not involve conscious recollection ("semantic memory") remains intact. The distinction between episodic and semantic memory is an instance of a broader distinction between conscious and unconscious mental operations, in humans and possibly other mammals also. However, the further subdivision of semantic memory into procedural and declarative lacks empirical basis, and indeed seems impossible to operationalize. Amnesics' difficulty in recollecting events (and partially learned facts) from before the onset of their disease (retrograde amnesia) is explicable in terms of interference between current events and prior events in similar contexts in patients who are unduly controlled by their current context. Consolidation theory is incoherent, and not needed to explain retrograde amnesia.
