 The book begins with a clear and compelling discussion of the interdependence of properties and laws, illustrated through the relations of alternative measures of length and alternative geometries. The examples are then used to make two general claims, first that within the structure of complex theories there are identifiable points that are definitions, and, second, that alternative systems of definitions and laws that account for the same “facts” say the same thing, have the same content. Reichenbach’s “definitions” are not purely verbal, they are “coordinative definitions” that specify physical procedures for determining the values of quantities. The point is reinforced with a distinction between differential and universal forces. Universal forces influence all objects in the same way (where theory specifies what a “way” is) and cannot be shielded against. Heat, for example, is a differential force because it affects bodies of different compositions differently, whereas gravity is a universal force. A space with curvature can be equally described as a space without curvature but with a universal force. Without explicit statement, Reichenbach takes for granted the view, later defended in Experience and Prediction (1938c), that talk in coordinative definitions about ordinary, middle-sized, nearby physical objects and processes is intelligible and legitimate. 