5. If the seeking of excitement, as such, is one of the prime pleasures of life, organized excitement in the form of interest is the directing and guiding principle of activity. At the outset of life interest is in the main involuntary and is aroused by the sights, sounds and happenings of the outer world. As time goes on, as the organism develops, as memories of past experiences become active, as peculiarities of personality develop, and as instincts reach activity, interest commences to take definite direction, to become canalized, so to speak. In fact, the development of interest is from the diffuse involuntary form of early childhood to a specialization, a condensation into definite voluntary channels. This development goes on unevenly, and is a very variable feature in the lives of all of us. Great ability expresses itself in a sustained interest; a narrow character is one with overdeformed, too narrow interest; failure is often the retention of the childish character of diffuse, involuntary interest. And the capacity to sustain interest depends not only on the special strength of the various abilities of the individual, but remarkably on his energy and health. Sustained "voluntary" interest is far more fatiguing than involuntary interest, and where fatigue is already present it becomes difficult and perhaps impossible. Thus after much work, whether physical or mental, during and after illness--especially in influenza, in neurasthenic states generally, or where there is an inner conflict--interest in its adult form is at a low ebb.