The _fourth_ law of association is, that the recurrence of associated objects depends, in a great measure, upon the _number_ of objects with which it may have been connected in past experience. If it has existed in combination with only _one_ object, that object will return associated with it; but in proportion as the number of its associates increases, the power of determining which will be its next companion diminishes. As an example of this fact may be mentioned the first hearing of a beautiful air by some particular person. The next time it is heard, the idea of this performer will be associated with the sounds; but after it has been sung by a great variety of persons, other circumstances would determine what conceptions this air would recall. It is very probable, in this case, that its notes would recall from among the associated scenes the friend most beloved, or some interesting circumstance that awakened emotion at the time the air was performed. The principal circumstances which operate in recalling associated ideas have now been pointed out. The next inquiry is, What are those objects and events which ordinarily are most frequently united in our _perceptions_, and therefore are most likely to return together in our _conceptions_? The most common connection of our ideas of perception are made by contiguity in _place_. Objects are continually passing before the eye, and they are not in single distinct objects, but in connected groups. Of course, when we perceive any object, we must necessarily observe its several relations to the things by which it is surrounded. If it is a building which meets the eye, it is impossible to observe it without at the same time perceiving the trees around it, the sky above it, and any other objects which are parts of the picture of which this is the prominent object. Of course, objects that are united in one complex picture before the eye when we gain our knowledge of them by perception, will ordinarily return together in our conceptions. Our ideas, also, are very much connected by contiguity as it respects _time_. When any two events occur at the same moment of time, or in such near connection that the conception of one remains until the other occurs, they ordinarily will recur together in our after conceptions of them. As an example of this may be mentioned the associations of a family who have been accustomed to close each Sabbath with music. As the still hour of this sacred evening drew on, wherever any wanderer might roam, it is probable that the notes of praise, so often connected with this season, would perpetually steal over the mind, bringing many another image of friends, and kindred, and home.