2. If sight depends upon the linking of the light of vision with the light leading progressively to the illumined object, then, by the very hypothesis, one intervening substance, the light, is indispensable: but if the illuminated body, which is the object of vision, serves as an agent operating certain changes, some such change might very well impinge immediately upon the eye, requiring no medium; this all the more, since as things are the intervening substance, which actually does exist, is in some degree changed at the point of contact with the eye [and so cannot be in itself a requisite to vision].
Those who have made vision a forth-going act [and not an in-coming from the object] need not postulate an intervening substance- unless, indeed, to provide against the ray from the eye failing on its path- but this is a ray of light and light flies straight. Those who make vision depend upon resistance are obliged to postulate an intervening substance.
The champions of the image, with its transit through a void, are seeking the way of least resistance; but since the entire absence of intervenient gives a still easier path they will not oppose that hypothesis.
So, too, those that explain vision by sympathy must recognize that an intervening substance will be a hindrance as tending to check or block or enfeeble that sympathy; this theory, especially, requires the admission that any intervenient, and particularly one of kindred nature, must blunt the perception by itself absorbing part of the activity. Apply fire to a body continuous through and through, and no doubt the core will be less affected than the surface: but where we are dealing with the sympathetic parts of one living being, there will scarcely be less sensation because of the intervening substance, or, if there should be, the degree of sensation will still be proportionate to the nature of the separate part, with the intervenient acting merely as a certain limitation; this, though, will not be the case where the element introduced is of a kind to overleap the bridge.
But this is saying that the sympathetic quality of the universe depends upon its being one living thing, and that our amenability to experience depends upon our belonging integrally to that unity; would it not follow that continuity is a condition of any perception of a remote object?
The explanation is that continuity and its concomitant, the bridging substance, come into play because a living being must be a continuous thing, but that, none the less, the receiving of impression is not an essentially necessary result of continuity; if it were, everything would receive such impression from everything else, and if thing is affected by thing in various separate orders, there can be no further question of any universal need of intervening substance.
Why it should be especially requisite in the act of seeing would have to be explained: in general, an object passing through the air does not affect it beyond dividing it; when a stone falls, the air simply yields; nor is it reasonable to explain the natural direction of movement by resistance; to do so would bring us to the absurdity that resistance accounts for the upward movement of fire, which on the contrary, overcomes the resistance of the air by its own essentially quick energy. If we are told that the resistance is brought more swiftly into play by the very swiftness of the ascending body, that would be a mere accidental circumstance, not a cause of the upward motion: in trees the upthrust from the root depends on no such external propulsion; we, too, in our movements cleave the air and are in no wise forwarded by its resistance; it simply flows in from behind to fill the void we make.
If the severance of the air by such bodies leaves it unaffected, why must there be any severance before the images of sight can reach us?
And, further, once we reject the theory that these images reach us by way of some outstreaming from the objects seen, there is no reason to think of the air being affected and passing on to us, in a progression of impression, what has been impressed upon itself.
If our perception is to depend upon previous impressions made upon the air, then we have no direct knowledge of the object of vision, but know it only as through an intermediary, in the same way as we are aware of warmth where it is not the distant fire itself that warms us, but the warmed intervening air. That is a matter of contact; but sight is not produced by contact: the application of an object to the eye would not produce sight; what is required is the illumination of the intervening medium; for the air in itself is a dark substance: If it were not for this dark substance there would probably be no reason for the existence of light: the dark intervening matter is a barrier, and vision requires that it be overcome by light. Perhaps also the reason why an object brought close to the eye cannot be seen is that it confronts us with a double obscuration, its own and that of the air.