A navigator in charge of the steering of aship, in addition to the compass and other aids, has to keep a constant look-out for rocks, some of which are visible, others submerged. The locality of these submerged rocks is as a rule well known and is marked on the chart, but from time to time we read that a ship has Struck on some uncharted rock or reef and has gone down, or at least has suffered some damage.

There is something analogous to this in the life of a human being. He has a course to steer, a goal to reach; he has rocks to encounter and keep clear of. As long as these are visible—as long as he is conscious, that is to say, of these rocks—it is an easier matter; but from time to time he strikes something which, so to speak, is not on his chart and yet profoundly influences his life. Disturbance of an emotional character may ensue, the root of which lies in the uncharted rocks of the sub-conscious mind.

This is merely a simile introduced by way of illustration to show the fundamental thought which runs through this treatise on the psychology of emotion.

The part which emotion—or, as older writers would say, passion—plays in human life is so obvious that the fact is barely worth mentioning. Without emotion life would resemble the existence of an oyster, fixed on its bed, immovable, impervious to any stimulation other than that which procures for it its daily diet of animalculae. It is in the quality of the emotional life that we find those oscillations of the personality which are familiar to us all.