8. Let us go deeper into the matter. If we only received the successive impressions without any means of referring them to the same object, to connect them in a common point, we should find no difference between the two cases of which we have been speaking. If a piece of white paper be placed before us, and, after turning our eyes aside for a moment, we find a blue paper in the same place, with the same dimensions, and after again turning our eyes aside we find a red paper: it is clear that it would be impossible for us to distinguish, by the mere succession of the visual impressions, whether the same paper has been differently colored in succession, or different papers have been substituted for the first. But if we keep our eyes on the place where the paper is, we see whether the paper is colored or changed. In the first case, the appearance of the new color will continue with the same sensation of the paper, unmoved, the transformation is made without our losing sight of it, and the paper receives the continued succession of its motions and positions under the hand of the one who colors it. We are then sure that the paper is the same, because there has been a continuity of sensation, or rather a connection of the different colors with a third, resulting from the situation of the paper and its motions, and from all that by which we know what is common to the first and the second. But if there is no new coloring of the paper, but a substitution of a differently colored paper, we see that the first paper is taken away; the whole order of the sensation is interrupted, and new sensations are presented. These last have no connection with the first; there is, consequently, for us a different _thing_. 9. This shows how the idea of substance with respect to bodies is generated in us, or, to speak more properly, how we apply the idea of substance to bodies. When we discover a link which unites the different sensations in one point, we call that in which they are united, substance. And as we meet in nature with many of these points which are independent of one another, we naturally say there are many corporeal substances.