Buffon thus distinguishes between "sensation" and "perception." "Sensation," he says, "is simply the activity of a sense, but perception is the pleasantness or unpleasantness of this sensation," "perceived by its being propagated and becoming active throughout the entire system." I have therefore several times, when translating from Buffon, rendered the word "_sentiment_" by "perception," and shall continue to do so. "I say," writes Buffon, "the pleasantness or unpleasantness, because this is the very essence of perception; the one feature of perception consists in perceiving either pain or pleasure; and though movements which do not affect us in either one or the other of these two ways may indeed take place within us, yet we are indifferent to them, and do not perceive that we are affected by them. All external movement, and all exercise of the animal powers, spring from perception; its action is proportionate to the extent of its excitation, to the extent of the feeling which is being felt.[94] And this same part, which we regard as the centre of sensation, will also be that of all the animal powers; or, if it is preferred to call it so, it will be the common _point d'appui_ from which they all take rise. The diaphragm is to the animal what the 'stock' is to the plant; both divide an organism transversely, both serve as the _point d'appui_ of opposing forces; for the forces which push upward those parts of a tree which should form its trunk and branches, bear upon and are supported by the 'stock,' as do those opposing forces, which drive the roots downwards. . . . . . . "Even on a cursory examination we can see that all our innermost affections, our most lively emotions, our most expansive moments of delight, and, on the other hand, our sudden starts, pains, sicknesses, and swoons--in fact, all our strong impressions concerning the pleasure or pain of any sensation--make themselves felt within the body, and about the region of the diaphragm. The brain, on the contrary, shows no sign of being a seat of perception. In the head there are pure sensations and nothing else, or rather, there are but the representations of sensations stripped of the character of perception; that is to say, we can remember and call to mind whether such and such a sensation was pleasant to us or otherwise, and if this operation, which goes on in the head, is followed by a vivid perception, then the impression made is perceived in the interior of the body, and always in the region of the diaphragm. Hence, in the foetus where this membrane is without use, there is no perception, or so little that nothing comes of it, the movements of the foetus, such as they are, being rather mechanical than dependent on sensation and will.