Not only was Herbart’s view of the idea very close to Leibniz’s view of the monad, but Herbart also borrowed the concept of apperception from Leibniz. According to Herbart, at any given moment, com- patible ideas gather in consciousness and form a group. This group of compatible ideas constitutes the apperceptive mass. Another way of looking at the apperceptive mass is to equate it with attention; that is, the apperceptive mass contains all ideas to which we are currently attending.
It is with regard to the apperceptive mass that ideas compete with each other. An idea outside the apperceptive mass (that is, an idea of which we are not conscious) will be allowed to enter the apperceptive mass only if it is compatible with the other ideas contained there at the moment. If the idea is not compatible, the ideas in the apperceptive mass will mobilize their energy to prevent the idea from entering. Thus, whether an idea is a new one derived from experience or one already existing in the unconscious, it will be permitted conscious expression only if it is compatible with the ideas in the apperceptive mass.