The mind informing the senses, instead of the senses informing the mind. There can be no doubt that the senses may be excited by the mind (from within, as well as from without). Macbeth saw the dagger of his imagination as distinctly as the dagger which he held in his hand. Malebranche declared that he heard the voice of God. Descartes thought he was followed by an invisible person, telling him to pursue his search for truth. Goethe says that, on one occasion, he met an exact counterpart of himself. Sir Walter Scott was fully persuaded that he had seen the ghost of the deceased Byron. All such hallucinations (due to mental disturbances) are of such stuff as dreams are made of.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894