There is, nevertheless, a considerable degree of subtlety in the relationship of the two principles. There may be good reasons {121} for treating them as distinct, but in their working they are inextricably combined. There can be no contiguity without similarity, and no similarity without contiguity. When, looking at a river, we pronounce its name, we are properly said to exemplify contiguity; the river and the name by frequent association are so united that each recalls the other. But mark the steps of the recall. What is strictly present to our view is the impression made by the river while we gaze on it. It is necessary that this impression should, by virtue of similarity or identity, re-instate the previous impression of the river, to which the previous impression of the name was contiguous. If one could suppose failure in the re-instatement of the former idea of the river, under the new presentation, there would be no opportunity given to the contiguous bond to come into operation. In that accumulation of the impressions of contiguous ideas, ending at last in a firm association, there must be a process of similarity to the extent of reviving the sum of the past at the instance of the present. This is a case of similarity that we give little heed to, because it is sure and unfailing; we concern ourselves more with what is liable to uncertainty, the acquired strength of the contiguous adhesion. Yet it strictly comes under the case of reproduction through similarity. Consider again, what may be called a case of Similarity proper, as when a portrait recalls the original. The sensuous effects possessed in common by the portrait and by its subject bring about a restoration of the idea of the subject, in spite of certain differences or discrepancies. The interest of this case is owing to the fact that a partial likeness, a likeness in unlikeness, will often reproduce a past idea; thus enabling us to assemble in the mind a number of things differing in some respects because they agree in other respects. This is not identifying a thing with itself, viewed at a former time, but assimilating one thing with other things placed far asunder in nature, and having many features of difference.