IMAGES DO NOT REACH US BY EFFLUENCE.
If these images do not reach us by some sort of effluence, why should the air be affected, and why should we ourselves be affected only as a result of the affection experienced by the air? If we felt only because the air had been affected before us, we would attribute the sensation of sight not to the visible object, but to the air located near us, as occurs with heat. In the latter case it is not the distant fire, but the air located near us which, being heated, then warms us; for the sensation of heat presupposes contact, which does not occur with vision. We see, not because the sense-object is imposed on the eye (but because the medium is illuminated); now it is necessary for the medium to be illuminated because the air by itself is dark. If the air were not dark, it would have no need of light; for (to effectuate vision) the obscurity, which forms an obstacle to vision, must be overcome by light. That is perhaps the reason why an object placed very near the eye is not seen; for it brings with it the darkness of the air, together with its own.