70. Condillac places the idea of extension in some things being presented to us outside of others. This, as he him self confesses, is verified in the sensation of color; therefore the sight of what is colored must produce the idea of extension. Condillac's subterfuge here is an exceedingly weak one. He pretends that it is necessary, in order to have the idea of extension, to have that of its limits. But first of all, we have shown from his own doctrine that these limits are perceived by the senses; besides it is a very strange pretension to attribute to sight the faculty of giving us the idea of unlimited extension, and to deny to it that of producing the idea of limitation; as if there did not by the very fact of our seeing what is extended, rise within us the idea of limitation, if from no other cause, from the very limitation of our organ; or as if an unlimited were not more inconceivable than a limited sensation. But suppose the limits not be perceived by the senses, does unlimited extension therefore cease to be extension? Is it not rather extension of the highest order? Does the idea of space without end, because unlimited, cease to be an idea of extension? 71. Two colored circles, one an inch, and the other a yard in diameter, are placed before our eyes; will the effect produced upon the retina be the same in both, abstraction made from all sensation of touch? Evidently not; experience shows the contrary, and the reason is founded on the laws of the reflection of light, and on mathematical principles. If the impressions are different, the difference will be perceived; therefore the difference of magnitude can be appreciated. We will now suppose some one in spite of reason and experience to persist in maintaining that the sensation of the two circles will be the same in order to make the extravagance, even the ridiculousness of this opinion palpable. Let us imagine the two circles to be of a red color, and terminated by a blue line; and now placing the less upon the greater circle so as to bring their centres together, we ask, will not the eye cast upon the figure see the less within the greater circle? Will not the blue line that terminates the circle of an inch in diameter be sure to be contained within the blue line that terminates the other circle of a yard in diameter? But what else is the perception of extension than the perception of some parts beyond others? Is it not to perceive the difference of magnitude, to perceive some greater than others, and containing them? Evidently it is. The sight therefore perceives magnitude; therefore it perceives extension.