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
is certain by experience that when we look at a near OBJECT with both
eyes, according as it approaches or recedes from us, we alter the
disposition of our eyes, by lessening or widening the interval between
the PUPILS. This disposition or turn of the eyes is attended with a
sensation, which seems to me to be that which in this case brings the
IDEA of greater or lesser distance into the mind.

17. Not that there is any natural or necessary connection between the
sensation we perceive by the turn of the eyes and greater or lesser
distance, but because the mind has by constant EXPERIENCE found the
different sensations corresponding to the different dispositions of the
eyes to be attended each with a different degree of distance in the
OBJECT: there has grown an habitual or customary connection between those
two sorts of IDEAS, so that the mind no sooner perceives the sensation
arising from the different turn it gives the eyes, In order to bring the
PUPILS nearer or farther asunder, but it withal perceives the different
IDEA of distance which was wont to be connected with that sensation; just
as upon hearing a certain sound, the IDEA is immediately suggested to the
understanding which custom had united with it.

18 Nor do I see how I can easily be mistaken in this matter. I know
evidently that distance is not perceived of itself. That by consequence
it must be