optics. I appeal to anyone's experience whether upon sight of an OBJECT he
computes its distance by the bigness of the ANGLE made by the meeting of
the two OPTIC AXES? Or whether he ever thinks of the greater or lesser
divergency of the rays, which arrive from any point to his PUPIL? Everyone
is himself the best judge of what he perceives, and what not. in vain
shall all the MATHEMATICIANS in the world tell me, that I perceive certain
LINES and ANGLES which introduce into my mind the various IDEAS of
DISTANCE, so long as I myself am conscious of no such thing.

13. Since, therefore, those ANGLES and LINES are not themselves perceived
by sight, it follows from sect. 10 that the mind doth not by them judge
of the distance of OBJECTS.

14. Secondly, the truth of this assertion will be yet farther evident to
anyone that considers those LINES and ANGLES have no real existence in
nature, being only an HYPOTHESIS framed by the MATHEMATICIANS, and by them
introduced into OPTICS, that they might treat of that science in a
GEOMETRICAL way.

15. The third and last reason I shall give for rejecting that doctrine is,
that though we should grant the real existence of those OPTIC ANGLES,
etc., and that it was possible for the mind to perceive them, yet these
principles would not be found sufficient to explain the PHENOMENA of
DISTANCE, as shall be shown hereafter.

16. Now, it being already shown that distance is suggested to the mind by
the mediation of some other IDEA which is itself perceived in the act of
seeing, it remains that we inquire what IDEAS or SENSATIONS there be that
attend VISION, unto which we may suppose the IDEAS of distance are
connected, and by which they are introduced into the mind. And FIRST, it