§2
Let us now consider briefly why persons who have not previously been irritable, should suddenly become irritable; who have not previously been hysterical, should suddenly become hysterical; who have not previously been in the habit of weeping, should at some time after reaching adult life, revert to that infantile habit.
The explanation of mental troubles of various kinds involves two factors. In the first place, any individual is capable of bearing a certain amount of conflict and a certain amount of repression. It is only when the accumulated force is more than he can control, that is when new determinants are added, that the symptoms begin to appear. He is like a steam engine in which as long as the steam is being used up in doing work, or as long as the safety valve is working efficiently when work is not being done, the boiler stands a pressure of 100lbs. very comfortably. If the safety valve gets jammed, and the energy cannot be transferred from the steam to the work, the pressure in the boiler rises higher and higher until it bursts from the joints and rivet-holes.
The second factor which determines the mode of expression of this out-burst of repressed energy is known as the law of regression. This means that if the adult outlet of energy becomes dammed up or is insufficient, the energy will flow through an earlier channel which has once been used. The individual will, in fact, revert to some method which he was wont to use in earlier years, or in infancy. It is true that this may be disguised and not recognised as an infantile mode of expression until it is looked into more closely. This question of regression, however, need not be more than touched upon here. It will be much more fully dealt with when we come to actual examples at a later stage.