Thirteen days: Joseph Delboeuf versus Pierre Janet on the nature of hypnotic suggestion. 
The problem of post-hypnotic suggestion was introduced in 1884.  Give a hypnotic subject the post-hypnotic command to return in 13 days.  Awake, the subject remembers nothing yet nonetheless fulfills the command to return.  How then does the subject count 13 days without knowing it?  In 1886, Pierre Janet proposed the concept of dissociation as a solution, arguing that a second consciousness kept track of time outside of the subject's main consciousness.  Joseph Delboeuf, in 1885, and Hippolyte Bernheim, in 1886, proposed an alternative solution, arguing that subjects occasionally drifted into a hypnotic state in which they were reminded of the suggestion.  This article traces the development of these competing solutions and describes some of Delboeuf's final reflections on the problem of simulation and the nature of hypnosis.