The mere psychologist will ever persevere in placing even the palpable causes of illusion beyond the reach of our inquiries.
Thus the rhapsodies of Lucretius were a series of professed fables, and the theories of Macrobius a tissue of capricious distinctions, as you may learn from his classification.
1st. ονειρος, somnium, dream. A figurative vision to be interpreted.
2nd. οραμα, vision. A vision which has afterwards been exactly fulfilled.
3rd. χρηματισμος, oraculum. An intimation in sleep of what we ought to do.
(I suppose as the shade of Hector appeared to Æneas, warning him, the night before, to escape from the flames of Troy.)
4th. ενυπνιον, insomnium. A sort of night-mare.
5th. φαντασμα, visus and incubus.
Here is a perfect jumble of classification, the first three only being vaunted as prophetic, or inspired; the fourth a night-mare; and the fifth, if it be any thing, a spectral illusion.
Others have deemed themselves mighty wise in discovering dreams to be the “action of intellect on itself.”