Educators typically think that one teaches evolution to develop students' conceptual understanding of evolution. It is assumed that if students understand evolution they will believe it. From a constructivist perspective it can be argued that understanding and belief, though related, are distinct concepts and that each is a potential goal for instruction. Though there are good reasons why belief should not be an instructional goal, achieving conceptual understanding requires that issues of belief be addressed. The point is that students are not likely to gain much understanding of something that they dismiss outright as unbelievable. What counts as believable for an individual rests on that person's worldview. This article argues that instruction on evolution can profitably begin with a dialogue on what counts as believable based on a study of the cultural history of Darwinism. The purpose of this strategy is to create in the classroom a shared meaning that certain fundamental questions are worth discussing and that the biological principles of evolution can contribute to that discussion.