In cases where neither knows anything of the Other, there are two persons if memory is used as the definition, but only one if the body is used. There is a regular gradation to the extreme of dual personality, through absent-mindedness, hypnosis, and sleepwalking. This makes a difficulty in using memory as the definition of personality. But it appears that lost memories can be recovered by hypnotism or in the course of psycho-analysis ; thus perhaps the difficulty is not insuperable.
Source: Religion and Science, 1935, chapt. 5
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