The central conceit of nanoscience is that size matters, and that the new material properties scientists see in nanomaterials are a result of changing scale. So to achieve both the practical goals of nanotechnology and to understand the behavior of nanoscale materials, the basic content of nanoscientists' reasoning must contain a notion of scale-dependence. Theories, concepts, and explanations arise out of scale-dependent thinking. Characterizing that kind of thought is central to understanding how reasoning works in nanoscience, and how nanoscientists achieve both scientific and extra-scientific goals.
I use the lessons that come out of this research on scale-dependent reasoning both to help nanoscientists to better understand their goals and methods, and to show philosophers of science how historical theories of scientific explanation, theory structure, and related topics have overlooked the dual roles of scale and synthesis as shapers of scientific reasoning.