several reasons. First, these levels may build on some of the previous restructurings described above and provide deeper explanation of many phenomena. Given that many students do not achieve those understandings, they do not have an appropriate foundation for constructing the next level of explanation.

Second, these new levels can interact and mutually support each other. For example, a deep understanding of cell theory and basic biological processes of living things actually calls for students to integrate atomic-molecular ideas into their analyses of living things. Without that foundation and level of analysis, many of the ideas of cell theory remain hard to explain or understand. Finally, adding these new levels calls for greater sophistication than many students have. For example, it requires that students understand the nature and purpose of explanatory models and how they are evaluated. That is, they are evaluated on the basis of their ability to explain a pattern of evidence rather than on whether they “look like” what is to be explained. If students do not have this kind of understanding, they may reject claims about atoms—such as that they are in constant motion—because these violate their commonsense impressions.

Mechanisms of Conceptual Change

One reason for distinguishing more fundamental, “revolutionary” conceptual changes from belief revision or conceptual elaboration is that these more profound forms of change may require a more complex coordination of a variety of learning mechanisms than more typical learning does. Most everyday learning involves knowledge enrichment and rests on an assumed set of concepts. For example, people use existing concepts to represent new facts, formulate new beliefs, make inductive or deductive inferences, and solve problems. Fundamental conceptual change, in contrast, involves coordinated adjustments of a variety of sorts in students’ network of concepts. The concepts of the new theory are ultimately organized and stated in terms of each other, rather than the concepts of the old theory, and there is no simple one-to-one correspondence between some concepts of the old and new theories. By what learning mechanisms, then, can students comprehend a genuinely new set of concepts and interrelations and come to prefer them to their initial set of concepts?

Acquiring New Knowledge Over an Existing Base of Concepts

First, the acquisition of new knowledge about the world (building on an initial base of concepts) is certainly an important part of the process of conceptual change (Carey, 1985; Chi, 1992; Case, 1997). For example, young children certainly won’t change their understanding of living things without

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