The difference between illusion and halucination

Interpreting a fleeting shadow as a person or hearing a voice in a babbling stream are illusions; but ‘if a man has the intimate conviction of actually perceiving a sensation for which there is no external object, he is in a hallucinatory state

 Hallucinations, like dreams, are independent of the senses: those who experience them might be said to be ‘dreaming while awake’.