Objects, he thinks, remind us of other objects resembling them, because we are accustomed to see like things together. But we are also accustomed to see like things separate. When two combinations incompatible with one another are both realised in familiar experience, it requires a very great preponderance of experience on one side to determine the association specially to either. We are also much accustomed to see unlike things together; I do not mean things contrasted, but simply unlike. Unlikeness, therefore, not amounting to contrast, ought to be as much a cause of association as likeness. Besides, the fact that when we see (for instance) a sheep, we usually see more sheep than one, may cause us, when we think of a sheep, to think of an entire flock; but it does not explain why, when we see a sheep with a black mark on its forehead, we are reminded of a sheep with a similar mark, formerly seen, though we never saw two such sheep together. It does not explain why a portrait makes us think of the original, or why a stranger whom we see for the first time reminds us of a person of similar appearance whom we saw many years ago. The law by which an object reminds us of similar objects which we have been used to see along with it, must be a different law from that by which it reminds us of similar objects which we have not been used to see along with it. But it is the same law by which it reminds us of dissimilar objects which we have been used to see along with it. The sight of a sheep, if it reminds us of a flock of sheep, probably by the same law of contiguity, reminds us of a meadow; but it must be by some other law that it reminds us of a single sheep previously seen, and of the occasion on which we saw that single sheep.