Even though "reals" are disrupted by other forces appearing to cause a change in the "reals" themselves, they are thought to be unchangeable. "Reals" tend to collide and struggle with one another so much so that each "real" fights for its own self-preservation (Selbsterhaltung). The way in which the soul helps to preserve itself from its outwardly perceived destruction is through Herbart's concept of Vorstellungen, or ideas or mental representations. These ideas were regarded as dynamic forces that Herbart attempted to explain by means of mathematical formulas. Newton's influence can be seen in Herbart's beliefs about how forces mechanically interact with one another in the world to affect perceptions of reality. The mechanics of ideas involved their ability to move in different ways, whether they be moving up into the conscious or delving down into the unconscious. Different ideas come into contact with each other and result in more complex ideas through the processes of blending, fusing, fading and combining in a multitude of approaches. It is evident Herbart thought that ideas were not precise imitations of the existing items in the world but that they were the direct consequence of the interactions of individuals’ experiences with the external environment. An individual can only gain all the facts and their associated truth by understanding how their mental representations combine and potentially inhibit or contribute to one another. Herbart believed ideas crossed a limen of consciousness, or a boundary between the conscious and the unconscious, as they became clearer and strong enough to preserve themselves against their struggle with other forces. The ideas powerful enough to break through to the conscious formed the apperceiving mass, or a congregation of similar and related ideas dominating the conscious at any given moment. Expounding upon Leibniz’s concept of pétites perceptions and the idea of apperception, Herbart believed the apperceiving mass to be crucial in selecting similar ideas from down in the unconscious to join its forces in the conscious. Although the individual is focusing all of his/her attention on those complex ideas a part of the apperceiving mass in the conscious, it is possible for ideas in the unconscious to combine with other ideas related to them and struggle to break through the limen into the conscious, disrupting the present ideas a part of the apperceiving mass. Apperception played a key role in Herbart's educational theory. He saw apperception as more pivotal in the classroom than sense-perception because focusing on a child's apperceiving mass in relation to the material being taught can inform teachers of how to implement the material in such a way as to direct the child's ideas and thoughts to attend to certain information.