Although the psychology of hallucination does not enter largely into the literature of this country, it may be gathered from the writings of our English psychologists that most of them recognise in hallucinations, illusions, percepts, and ideas a family resemblance; but the points of dissimilarity among these processes have not, in my opinion, received their due measure of consideration. I propose, therefore, to describe the ordinary psychology of these processes, drawing especial attention to their points of dissimilarity; and I will endeavour to show that, while their resemblance is mainly psychological, their difference is mainly physiological.