When the mind is in a state of emotion, this state is always either pleasurable or painful. _Desire_ relates to the attainment of some object which will be the cause of pleasurable emotions, or else to the avoidance of something which will cause painful emotions. This desire for pleasure and for the avoidance of pain is the mainspring of all mental activity; for when it is not in existence, neither the powers of the mind or of the body are called into exercise. There are various sources of enjoyment or causes of pleasurable emotion to the mind of man, the most important of which will now be pointed out. The _first_ cause of enjoyment at the commencement of existence is that of _sensation_.[2] This, at first, is small in amount compared with what it becomes when association lends its aid to heighten sensitive enjoyment. The light of day, the brilliancy of color, the sweetness of perfume, the gratification of taste and touch, the magic influence of sound, and the pleasure resulting from muscular activity, are probably the chief sources of enjoyment to the infant mind. As life advances, all these modes of sensitive gratification become connected with others of an intellectual and moral nature, so that at mature years it is difficult to determine how much of the enjoyment we derive from the senses is the result of association, and how much is simply that of sensation. Another source of happiness to the human mind is the simple exercise of its intellectual powers. This includes all the pleasures derived from the exercise of taste and the imagination; all the more profitless exercises of reverie and castle-building; all the activity of mind employed in contriving, inventing, and bringing to pass the various projects for securing good to ourselves and others; and all those charming illusions which so often give transient delight, but burst like bubbles in the grasp. Another source of enjoyment is the exercise of physical and moral power. This love of power is one of the earliest principles which is developed in the human mind. The exercise of the muscles in producing changes in its own material frame or in surrounding objects is a source of constant pleasure to the infant mind. There are few who have reared a child through the period of infancy but can recollect the times that this new species of delight was manifested, as, with his hand raised before his eyes, he watched its various motions, and learned his own power to control them. This love of power continually displays itself in the sports and pursuits of childhood. To project the pebble through the air; to drive the hoop; to turn the windmill; to conduct some light stream from its channel; to roll the rock from the mountain cliff--these and many others are the varied modes by which childhood exhibits its love of physical power.