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Ideas of Sensation often changed by the Judgment.       §8. We are farther to consider concerning Perception, that the
Ideas we receive by sensation, are often in grown People alter’d by the
Judgment, without our taking notice of it. When we set before our
Eyes a round Globe, of any uniform colour, v.g. Gold, Alabaster, or
Jet,’tis certain, that the Idea thereby imprinted in our Mind, is of a
flat Circle variously shadow’d, with several degrees of Light and
Brightness coming to our Eyes. But we having by use been accus-
tomed to perceive, what kind of appearance convex Bodies are wont
to make in us; what alterations are made in the reflections of Light, by
the difference of the sensible Figures of Bodies, the Judgment pres-
ently, by an habitual custom, alters the Appearances into their
Causes: So that from that, which truly is variety of shadow or colour,
collecting the Figure, it makes it pass for a mark of Figure, and frames
to it self the perception of a convex Figure, and an uniform Colour;
when the Idea we receive from thence, is only a Plain variously col-
our’d, as is evident in Painting. To which purpose I shall here insert
a Problem of that very Ingenious and Studious promoter of real Know-
ledge, the Learned and Worthy Mr. Molineux , which he was pleased to
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send me in a Letter some Months since; and it is this: Suppose a Man
born blind, and now adult, and taught by his touch to distinguish between a
Cube, and a Sphere of the same metal, and nighly of the same bigness, so as to
tell, when he felt one and t’other, which is the Cube, which the Sphere.
Suppose then the Cube and Sphere placed on a Table, and the Blind Man to be
made to see. Quaere, Whether by his sight, before he touch’d them, he could
now distinguish, and tell, which is the Globe, which the Cube
. To which the
acute and judicious Proposer answers: Not. For though he has obtain’d
the experience of, how a Globe, how a Cube affects his touch; yet he has not
yet attained the Experience, that what affects his touch so or so, must affect
his sight so or so; Or that a protuberant angle in the Cube, that pressed his
hand unequally, shall appear to his eye, as it does in the Cube. I agree with
this thinking Gent. whom I am proud to call my Friend, in his
answer to this his Problem; and am of opinion, that the Blind Man,
at first sight, would not be able with certainty to say, which was
the Globe, which the Cube, whilst he only saw them: though he
could unerringly name them by his touch, and certainly distinguish
them by the difference of their Figures felt. This I have set down,
and leave with my Reader, as an occasion for him to consider, how
much he may be beholding to experience, improvement, and ac-
quired notions, where he thinks, he has not the least use of, or help
from them: And the rather, because this observing Gent. farther
adds, that having upon the occasion of my Book, proposed this to divers very
ingenious Men, he hardly ever met with one, that at first gave the answer to
it, which he thinks true, till by hearing his reasons they were convinced.
Locke Hum II, 9, §8, pp. 145-146