5. In what does sensation consist? What is its internal nature? We only know that it is a modification of our being, and that we cannot explain it. No words suffice to convey an idea of a sensation to one who has never experienced it. The man born blind may listen to all that philosophers have said and written on light and colors, but can never imagine what light and colors are. Experience is the only teacher here; and thus, if we suppose a man's senses to be changed so that green appears to him purple, and purple green, notwithstanding his constant communication with other men, he will never be freed from his error, and he will never suspect that during his whole life he has made use of the words green and purple in a different sense from other men. 6. Analogy and nature incline us to believe that brutes are not mere machines, but that they also have sensations. The vast scale over which irrational beings are distributed, shows that the faculty of feeling is spread over the universe in different degrees, and with a wonderful profusion. Our experience is confined to the globe in which we live; but are the limits of sensitive life the same as those of our experience? Even on this globe our observation is confined to what the imperfection of our senses, and the instruments which we use, permit; but how far is the chain of life prolonged? Where is its term? Is there not some participation of this mysterious faculty in those beings which we hold to be inanimate? Is the universe, as Leibnitz pretends, composed of a collection of monads endowed with a certain perception? This is indeed an unfounded hypothesis; but since our means of observation are so limited, we should be cautious how we assign boundaries to the realm of life. 7. We ordinarily speak of the faculty of sensation as of something belonging to a very inferior order; so it is, in fact, if compared with intellectual faculties; but this does not prevent it, in itself considered, from being a wonderful phenomenon, of a nature to astonish and confound all who meditate upon it. Sensation! With this word alone we pass an immense gap in the scale of beings. What is the non-sensible compared with the sensible? The insensible is, but experiences not that it is; there is nothing in it but itself: the sensible experiences that it is; and there is in it something besides itself, all that it feels, all that is represented in it. The insensible, although surrounded with beings, is in complete isolation,--in solitude; the sensible, although alone, may be in a world of infinitely varied representations.