 Constructive empiricists might maintain that, for working scientists, the real importance of scientific theories is that they are a factor in experimental design. They contrast this with the traditional picture presented by philosophy of science. According to the traditional picture, the main goal of scientific practice is to discern the fundamental structure of the world, and experimentation simply is used to determine whether theories should be taken to be true, and hence as contributing to our knowledge of the fundamental structure. The constructive empiricist, in contrast, suggests that the reason a scientist turns to a theory is that experimental design is difficult, and theories are needed to guide experimental inquiry. But what scientists are really aiming to discover, according to the constructive empiricist, are “facts about the world—about the regularities in the observable part of the world” (van Fraassen 1980, 73).