Let a man be always fore-armed against the unforeseen.  Let Emile run about barefoot all the year round, upstairs, downstairs, and in the garden.  Far from scolding him, I shall follow his example; only I shall be careful to remove any broken glass.  I shall soon proceed to speak of work and manual occupations.  Meanwhile, let him learn to perform every exercise which encourages agility of body; let him learn to hold himself easily and steadily in any position, let him practise jumping and leaping, climbing trees and walls.  Let him always find his balance, and let his every movement and gesture be regulated by the laws of weight, long before he learns to explain them by the science of statics.  By the way his foot is planted on the ground, and his body supported on his leg, he ought to know if he is holding himself well or ill.  An easy carriage is always graceful, and the steadiest positions are the most elegant.  If I were a dancing master I would refuse to play the monkey tricks of Marcel, which are only fit for the stage where they are performed; but instead of keeping my pupil busy with fancy steps, I would take him to the foot of a cliff.  There I would show him how to hold himself, how to carry his body and head, how to place first a foot then a hand, to follow lightly the steep, toilsome, and rugged paths, to leap from point to point, either up or down.  He should emulate the mountain-goat, not the ballet dancer.

As touch confines its operations to the man’s immediate surroundings, so sight extends its range beyond them; it is this which makes it misleading; man sees half his horizon at a glance.  In the midst of this host of simultaneous impressions and the thoughts excited by them, how can he fail now and then to make mistakes?  Thus sight is the least reliable of our senses, just because it has the widest range; it functions long before our other senses, and its work is too hasty and on too large a scale to be corrected by the rest.  Moreover, the very illusions of perspective are necessary if we are to arrive at a knowledge of space and compare one part of space with another.  Without false appearances we should never see anything at a distance; without the gradations of size and tone we could not judge of distance, or rather distance would have no existence for us.  If two trees, one of which was a hundred paces from us and the other ten, looked equally large and distinct, we should think they were side by side.  If we perceived the real dimensions of things, we should know nothing of space; everything would seem close to our eyes.

The angle formed between any objects and our eye is the only means by which our sight estimates their size and distance, and as this angle is the simple effect of complex causes, the judgment we form does not distinguish between the several causes; we are compelled to be inaccurate.  For how can I tell, by sight alone, whether the angle at which an object appears to me smaller than another, indicates that it is really smaller or that it is further off.