It has been demonstrated earlier that the eye perceives none of the visible objects that occupy the same air with it ... unless the following conditions are met, namely: (1) that there be some space between the eye and object, (2) that the object face the eye —i.e., that a straight line can be imagined extended between any point on the surface of the visible object perceived by the eye and some point on the surface of the eye, (3) that the object possess some illumination, (4) that it have some [perceptible] size with respect to the eye's sense-capacity, (5) that the aereal medium be continuous and transparent and that there not be any opaque body in it, and (6) that visible object block sight—i.e., that there be no transparency in it, or if there is, that it be more opaque than the air intervening between it and the eye; but this can only happen with color or the like.

Ibn al-Haytham, Alhacen's Theory of Visual Perception, tr. A. Mark Smith, p. 390 (8.1)

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