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74 THE FOURFOLD ROOT. [CHAP. IV.
one object ; so direct is the act of causal apprehension in
the Understanding.
We have not space enough here to refute one by one the
physiological explanations of single vision which have been
attempted; but their fallacy is shown by the following
considerations :
1. If seeing single were dependent upon an organic
connection, the corresponding points in both retinas, on
which this phenomenon is shown to depend, would corre
spond organically, whereas they do so in a merely geo
metrical sense, as has already been said. For, organically
speaking, the two inner and two outer corners of the eyes
are those which correspond, and so it is with the other
parts also ; whereas for the purpose of single vision,
it is
the right side of the right retina which corresponds to the
right side of the left retina, and so on, as the phenomena
just described irrefutably show. It is also precisely on
account of the intellectual character of the process, that
only the most intelligent animals, such as the higher
mammalia and birds of prey more especially owls have
their eyes placed so as to enable them to direct both optic
axes to the same point.
2. The hypothesis of a confluence or partial intersection
of the optic nerves before entering the brain, originated by
Newton, 1 is false, simply because it would then be impos
sible to see double by squinting. Vesalius and Caesal-
pinus besides have already brought forward anatomical
instances in which subjects saw single, although neither
fusion nor even contact of the optic nerves had taken
place. A final argument against the hypothesis of a mixed
impression is supplied by the fact, that on closing our right
eye firmly and looking at the sun with our left, the bright
image which persists for a time is always in the left, never
in the right, eye : and vice versa.
1
Newton,
"
Optics." Query 15.