Such being the state of affairs, suppose a second flash of red light to follow the first. If there were no memory of the latter, the state of the mind on the second occasion would simply be a repetition of that which occurred before. There would be merely another impression. But suppose memory to exist, and that an idea of the first impression is generated; then, if the supposed sentient being were like ourselves, there might arise in his mind two altogether new impressions. The one is the feeling of the _succession_ of the two impressions, the other is the feeling of their _similarity_. Yet a third case is conceivable. Suppose two flashes of red light to occur together, then a third feeling might arise which is neither succession nor similarity, but that which we call _co-existence_. These feelings, or their contraries, are the foundation of everything that we call a relation. They are no more capable of being described than sensations are; and, as it appears to me, they are as little susceptible of analysis into simpler elements. Like simple tastes and smells, or feelings of pleasure and pain, they are ultimate irresolvable facts of conscious experience; and, if we follow the principle of Hume's nomenclature, they must be called _impressions of relation_. But it must be remembered, that they differ from the other impressions, in requiring the pre-existence of at least two of the latter. Though devoid of the slightest resemblance to the other impressions, they are, in a manner, generated by them. In fact, we may regard them as a kind of impressions of impressions; or as the sensations of an inner sense, which takes cognizance of the materials furnished to it by the outer senses. Hume failed as completely as his predecessors had done to recognise the elementary character of impressions of relation; and, when he discusses relations, he falls into a chaos of confusion and self-contradiction. In the _Treatise_, for example, (Book I., Sec. iv.) resemblance, contiguity in time and space, and cause and effect, are said to be the "uniting principles among ideas," "the bond of union" or "associating quality by which one idea naturally introduces another." Hume affirms that-- "These qualities produce an association among ideas, and upon the appearance of one idea naturally introduce another." They are "the principles of union or cohesion among our simple ideas, and, in the imagination, supply the place of that inseparable connection by which they are united in our memory. Here is a kind of _attraction_, which, in the mental world, will be found to have as extraordinary effects as in the natural, and to show itself in as many and as various forms. Its effects are everywhere conspicuous; but, as to its causes they are mostly unknown, and must be resolved into _original_ qualities of human nature, which I pretend not to explain."--(I. p. 29.)