"If some one walking in the country hears a dog bark he perceives first a
sound: this is the act of hearing.

"He will distinguish that this sound is produced by the barking of a dog;
this is the act of understanding.

"Reflection will lead him then to think that a house or a human being is
near, for a dog goes rarely alone.

"If the things which are presented to our sight are complex, those which
strike our ears are summed up in one word, sound, which has only one
definition, the quality of the sound.

"Then follow the innumerable categories of sound that we distinguish only
by means of comprehension and reflection, rendered so instinctive by
habit that we may call them automatic, so far as those which relate to
familiar sounds.

"The example which we have just given is a proof of this fact.

"Let us add that this habit develops each sensitive faculty to its
highest degree.

"The inhabitants of the country can distinguish each species of bird by
listening to his song; and the hermits, the wanderers, those who live
with society on a perpetual war footing, perceive sounds which would not
strike the ears of civilized people.

"Approximation is also one of the stones by whose aid we construct the