Herein, according to the Herbartian school, lies the importance of directing the process of apperception by judicious selection of the materials which are to constitute the experience of the child. As the mind, in this view is simply built up entirely out of the ideas which it has received, the kinds of ideas presented to it and the order in which they come are of the utmost moment in the work of education. In the pleasure of the process of apperception lies the great fact of interest. Interest depends on what is already in the mind. Interest and knowledge react on each other.
Interest stimulates voluntary attention and sustains involuntary.
It thus ties at the root of the mental activity of observation. It determines what we shall see and also what we shall desire and will. Ignorance is really the main factor in vice. Connected with the insistence on the psychological agencies of apperception and interest is the Herbartian principle of correlation and the five formal steps of instruction. The former should, according to the school, govern the drawing up of the curriculum.