Here I pause for one moment, to exhort the reader never to pay any
attention to his understanding when it stands in opposition to any
other faculty of his mind. The mere understanding, however useful and
indispensable, is the meanest faculty in the human mind, and the most to
be distrusted; and yet the great majority of people trust to nothing else;
which may do for ordinary life, but not for philosophical purposes. Of this
out of ten thousand instances that I might produce, I will cite one. Ask of
any person whatsoever, who is not previously prepared for the demand by
a knowledge of perspective, to draw in the rudest way the commonest
appearance which depends upon the laws of that science; as for instance, to
represent the effect of two walls standing at right angles to each other,
or the appearance of the houses on each side of a street, as seen by a
person looking down the street from one extremity. Now in all cases, unless
the person has happened to observe in pictures how it is that artists
produce these effects, he will be utterly unable to make the smallest
approximation to it. Yet why? For he has actually seen the effect every day
of his life. The reason is--that he allows his understanding to overrule
his eyes. His understanding, which includes no intuitive knowledge of the
laws of vision, can furnish him with no reason why a line which is known
and can be proved to be a horizontal line, should not _appear_ a horizontal
line; a line that made any angle with the perpendicular less than a right
angle, would seem to him to indicate that his houses were all tumbling down
together. Accordingly he makes the line of his houses a horizontal line,
and fails of course to produce the effect demanded. Here then is one
instance out of many, in which not only the understanding is allowed to
overrule the eyes, but where the understanding is positively allowed to
obliterate the eyes as it were, for not only does the man believe the
evidence of his understanding in opposition to that of his eyes, but,
(what is monstrous!) the idiot is not aware that his eyes ever gave such