Enneads IV.5: Psychological Questions--III
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.
USELESSNESS OF AIR AS TRANSMITTING MEDIUM PROVED FROM SIGHT OF OBJECTS AT NIGHT.
3. A strong proof that the forms of sense-objects are not seen merely because the air, on being affected, transmits them by relays from point to point, is that even in darkness the fire, the stars, and their forms may be seen. In this case no one would claim that the forms of the objects, being impressed on the obscure air, are transmitted to the eye; otherwise, there would be no obscurity, as the fire, while transmitting its form, would illuminate. Indeed, in the profound obscurity in which the light of the stars is not seen, the fire of signals and of light-houses may be perceived. Should any one, in opposition to the testimony of his senses, claim that even in this case the fire penetrates the air, he should be answered by having it pointed out to him that in that case human vision should distinguish the smallest objects which are in the air, instead of being limited to the perception of the fire. If then we see what is beyond a dark medium, it would be much better seen without any medium whatever.
ABSENCE OF MEDIUM WOULD INTERFERE WITH VISION ONLY BY DESTROYING SYMPATHY.
It might indeed be objected that without medium, vision ceases. This occurs not because of the lack of medium, but because the sympathy of the (universal) organism is in such a case destroyed since a medium presupposes that all the parts of this organism together form but a single being. It would indeed seem to be a general condition necessary for sensation that the universal organism be sympathetic with itself; otherwise, no one thing could participate in the power of any other thing that might happen to be very distant.