Admitting, then, that some contact with an object is necessary for knowing it, the question of a medium falls to the ground in the case of things identified by any form of touch; but in the case of sight-we leave hearing over for the present- we are still in doubt; is there need of some bodily substance between the eye and the illumined object?
No: such an intervening material may be a favouring circumstance, but essentially it adds nothing to seeing power.
!Dense bodies, such as clay, actually prevent sight; the less material the intervening substance is, the more clearly we see; the intervening substance, then, is a hindrance, or, if not that, at least not a help.
It will be objected that vision implies that whatever intervenes between seen and seer must first [and progressively] experience the object and be, as it were, shaped to it; we will be reminded that [vision is not a direct and single relation between agent and object, but is the perception of something radiated since] anyone facing to the object from the side opposite to ourselves sees it equally; we will be asked to deduce that if all the space intervening between seen and seer did not carry the impression of the object we could not receive it.
But all the need is met when the impression reaches that which is adapted to receive it; there is no need for the intervening space to be impressed.If it is, the impression will be of quite another order: the rod between the fisher's hand and the torpedo fish is not affected in the same way as the hand that feels the shock.And yet there too, if rod and line did not intervene, the hand would not be affected- though even that may be questioned, since after all the fisherman, we are told, is numbed if the torpedo merely lies in his net.
The whole matter seems to bring us back to that sympathy of which we have treated.If a certain thing is of a nature to be sympathetically affected by another in virtue of some similitude between them, then anything intervening, not sharing in that similitude, will not be affected, or at least not similarly.If this be so, anything naturally disposed to be affected will take the impression more vividly in the absence of intervening substance, even of some substance capable, itself, of being affected.
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.