_Liberty_ is the experimental condition for studying the phenomena of the child's attention. It will be enough to remember that the stimuli of infant attention, being mainly sensory, have a powerful physiological concomitant of "accommodation" in the organs of sense; an accommodation, physiologically incomplete in the young child, which requires to develop itself according to Nature. An object not adapted to become a useful stimulus to the powers of accommodation in process of development would not only be incapable of sustaining attention as a psychical fact, but would also, as a physiological fact, weary or actually injure the organs of accommodation such as the eye and ear. But the child who _chooses_ the objects, and perseveres in their use with the utmost intensity of attention, as shown in the muscular contractions which give mimetic expression to his face, evidently experiences _pleasure_, and pleasure is an indication of healthy functional activity; it always accompanies exercises which are useful to the organs of the body. Attention also requires a _preparation_ of the ideative centers in relation to the external object for which it is to be demanded: in other words, an internal, psychical "adaptation." The cerebral centers should be excited in their turn by an internal process, when an external stimulus acts. Thus, for instance, any one who is expecting a person, sees him arriving from a considerable distance; not only because the person presents himself to the senses, but because he was "expected." The distant figure claimed attention because the cerebral centers were already excited to that end. By means of similar activities a hunter is conscious of the slightest sound made by game in woods. In short, two forces act upon the cerebral cell, as upon a closed door: the external sensory force which knocks, and the internal force which says: Open. If the internal force does not open, it is in vain that the external stimulus knocks at the door. And then the strongest stimuli may pass unheeded. The absent-minded man may step into a chasm. The man who is absorbed in a task may be deaf to a band playing in the street.