8. Sensations often changed by the judgment.
We are further to consider concerning perception, that the ideas we
receive by sensation are often, in grown people, altered 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, it is certain that the idea
thereby imprinted on our mind is of a flat circle, variously shadowed, with several degrees of light and brightness
coming to our eyes. But we having, by use, been accustomed 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 presently, by an habitual custom, alters the appearances into their causes. So that
from that which is truly variety of shadow or colour, collecting the figure, it makes it pass for a mark of figure,
and frames to itself the perception of a convex figure and an uniform colour; when the idea we receive from
thence is only a plane variously coloured, as is evident in painting. To which purpose I shall here insert a problem
of that very ingenious and studious promoter of real knowledge, the learned and worthy Mr. Molyneux, which he
was pleased to 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 the other, which is the cube, which the sphere. Suppose then the cube
and sphere placed on a table, and the blind man be made to see: quære, whether by his sight, before he touched
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 obtained the experience of how a globe, how a cube affects his touch,
yet he has not yet obtained 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 gentleman, whom I am proud to call my friend, in his answer to this 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 beholden to experience, improvement, and acquired notions,
where he thinks he had not the least use of, or help from them. And the rather, because this observing gentleman
further 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."
