 According to Shepherd, the idea of the continued existence of self has its origin in the perception that the sensation of life is interrupted by dreamless sleep and then resumed; we refer the renewal as effect to a cause that continues to exist. The cause is identified as the conjunction of the power to sustain life and the powers of memory and sensation. The power of memory is the ability to form ideas of memory. Ideas of memory are complex. On Shepherd’s analysis they contain: fainter sensations attended by the idea of the cause of their originals’ being remote, the perception of a lapse of time, and the perception of one’s own existence going on between the moment of original impression and the present faint sensations. This explanation is evidently so far incomplete. The main difficulty is to explain the perception of a lapse of time. 