The literature of science speaks of many things in the heavens and on earth that neither the man in the street nor the scientist in his laboratory, can directly see with his eyes or touch with his fingers. Scientific hypotheses refer to atoms and genes, cosmic rays and electrostatic charges. No one can deny the importance of these hypothetical constructions; without them we could neither understand nor manage the world which we directly experience. They simplify the system of laws that relate observable variables to one another. They suggest further research and enable us to predict what will be found if certain operations are carried out. The sporadic and disjointed collection of observed objects and events, and the occurrence of the unusual ones which we call illusions, are explained by correlating them with a hypothetical physical or physiological state of affairs that has order and spatio-temporal continuity.