In this sequence of philosophical essays about natural science, Nancy Cartwright argues that fundamental explanatory laws, the deepest and most admired successes of modern physics, do not in fact describe the regularities that exist in nature. Yet she is not `anti-realist’. Rather, she draws a novel distinction, arguing that theoretical entities, and the complex and localized laws that describe them, can be interpreted realistically, but that the simple unifying laws of basic theory cannot.