The mechanical principle of action and reaction is another which can be traced in mental as well as in physical phenomena. Mental excitement is often succeeded by mental depression, "after pleasure follows pain." The power of mental self-guidance and self-education is largely dependent upon the two well known scientific principles of latent energy, and action and reaction. We are able to liberate energy, not only in cases where it will influence inanimate matter but also ourselves. The principle of self-guidance is not restricted to living creatures, nor is self-regulation limited to mental power. The principle of self-regulation operates in clocks, watches, musical boxes, the governors of steam engines, water regulators, gas regulators, &c., &c., and upon an immense scale in the movements of the heavenly {108} bodies. With the electric locomotive, the greater the load it has to draw, or the steeper the incline it has to ascend, the more strongly does it exert its strength, up to the full limits of its power. Neither in physical nor in mental actions can a body or force usually act directly upon itself to change its state whether of activity or rest. In both classes of cases however we meet with plenty of instances where, a body by an almost imperceptible expenditure of energy on its own part either alters some surrounding conditions, or excites a powerful liberation of energy in another body which then reacts upon it to change its state. In this way the action of clock-work in the self-exploding apparatus of a torpedo liberates at a particular moment a spring, and causes an explosion which destroys the apparatus. Similarly, whilst a man, in many cases, is unable to _directly_ alter his mental state, to increase or diminish his mental activity, to cause sleep, &c., he is able _indirectly_ to change his mental condition by drinking stimulants or by adopting means of self-education; and to induce sleep by means of opium, suitable exercise, &c.