For a long time attempts have been made to discover definite relations between external and internal stimuli occurring during sleep and coincident dreams, as also between dreams and the more or less momentous happenings preceding them during waking hours. Such attempts as have been made to prove obvious connections have mostly failed, and for reasons which for the first time are set forth by Freud. At the same time, he has demonstrated the subtile evasions and tortuous routes which pertain to the operations of stimuli on the way to dream-consciousness, whether they be sensory or purely physical, as well as the indefinite period of time over which reminiscences, which are concerned in the evolution of the dream, may extend.