117. There is unity where there is no division, when in the thing that is one no others can be distinguished, when it admits no negative judgment. Nothing of all this is observed in the external world; but a constant experience presents directly the contrary. 118. In the external world division is visible, palpable; there is no other unity than that of order, of direction to an end; besides this, all is multiplicity. The only medium by which we are placed in communication with the external world are the senses, and they encounter multiplicity on every side--sensations distinct in number, diverse in species, graduated in a thousand different ways, distributed into infinite groups, which, although they are connected in this or that point, may be divided and are divided in a thousand others. 119. Multiplicity is as truly revealed by the testimony of the senses as the very existence of objects. If we deny the competency of their testimony in the first, we must deny it also in the second. They not only tell us that such a body exists, but that it is not another body. We know nothing with more certainty than that an external object corresponds to a sensation, that the objects of two distinct sensations are distinct. To say that the senses are not good judges in this matter, because they are limited to mere sensation, and consequently cannot judge of the objects of the sensation, is to appeal to idealism, for by the same reason we may assert that the senses, limited to mere sensation, cannot give us certainty of the existence of their respective objects. 120. To establish unity outside of ourselves is to annihilate the corporeal world. The idea of extension contradicts unity. In that which is extended some parts are not the others. This is evident, and whoever attempts to doubt it attacks the basis of the certainty of geometry. If the world is something real, it is extended; if it is not extended, we cannot be certain that it is any thing real. We have the same certainty of its extension as of its existence. Its very existence is manifested by the extension presented to our senses. If, then, this extension does not exist, sensations are a mere internal phenomenon, a pure illusion, in so far as we attribute to them a correspondence to the exterior.