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63 THE FOURFOLD ROOT. [CHAP, IV.
gends forth its rays towards all sides in a rectilinear direc
tion, the rays from its upper extremity cross those from its
lower extremity in the narrow aperture of the pupil, by
which the former impinge upon the bottom, the latter
upon the top, those projected from the right side upon the
left, and vice versa. The refracting apparatus of the eye,
which consists of the humor aqueus, lens, et corpus vitreum,
only serves to concentrate the rays of light proceeding from
the object, so as to find room for them on the small space
of the retina. Now, if seeing consisted in mere sensation,
we should perceive the impression of the object turned
but in that case
upside down, because we receive it thus ;
we should perceive it as something within our eye, for we
should stop short at the sensation. In reality, however,
the Understanding steps in at once with its causal law, and
as it has received from sensation the datum of the direc
tion in which the ray impinged upon the retina, it pursues
that direction retrogressively up to the cause on both
lines ; so that this time the crossing takes place in the oppo
site direction, and the cause presents itself upright as an
external object in Space, i.e. in the position in which it
originally sent forth its rays, not that in which they reached
the retina (see fig. 1). The purely intellectual nature of
this process, to the exclusion of all other, more especially of
physiological, explanations, may also be confirmed by the