These reserves once made, it remains to remark, that the objects we perceive very rarely present themselves in a state of perfect isolation. On the contrary, they are brought near to other objects by manifold relations of resemblance, of difference, or of connection in time or space; and, further, they are compared with the ideas which define them best. We do not have consciousness of an object, but of the relations existing between several objects. Relation is the new state produced by the fact that one perceives a plurality of objects, and perceives them in a group. Show me two colours in juxtaposition, and I do not see two colours only, but, in addition, their resemblance in colour or value. Show me two lines, and I do not see only their respective lengths but their difference in length. Show me two points marked on a white sheet of paper, and I do not see only the colour, form, and dimension of the points, but their distance from each other. In our perceptions, as in our conceptions, we have perpetually to do with the relations between things. The more we reflect, the more we understand things, the more clearly we see their relations; the multiplication of relations is the measure of the depth of cognition.[25] The nature of these relations is more difficult to ascertain than that of objects. It seems to be more subtle. When two sounds make themselves heard in succession, there is less difficulty in making the nature of these two sounds understood than the nature of the fact that one occurs before the other. It would appear that, in the perception of objects, our mind is passive and reduced to the state of reception, working like a registering machine or a sensitive surface, while in the perception of relations it assumes a more important part. Two principal theories have been advanced, of which one puts the relations in the things perceived, and the other makes them a work of the mind. Let us begin with this last opinion. It consists in supposing that the relations are given to things by the mind itself. These relations have been termed categories. The question of categories plays an important part in the history of philosophy. Three great philosophers, Aristotle, Kant, and Renouvier have drawn up a list, or, as it is called, a table of them, and this table is very long. To give a slight idea of it, I will quote a few examples, such as time, space, being, resemblance, difference, causality, becoming, finality, &c.