In 1890 the American psychologist and philosopher William James (1842-1910) defined hallucinations as follows: "An hallucination is a strictly sensationalform of consciousness, as good and true a sensation as if there were a real object there. The object happens not to be there, that is all."
References
James, W. (1952). The principles of psychology. Great books of the western world no. 5i.Edited by Hutchins, R.M. London: Encyclopaedia Britannica.
Dictionary of Hallucinations. J.D. Blom. 2010.