Abstract

In this paper, I argue that the conceptual changes that occurred in the structure of physical knowledge during the second half of the 19th century, are reflected by the concept of the “primary ideal object” and its implicit definition within appropriate systems of statements, called a “nucleus of a branch of physics”. Within an NBP focus shifts away from discovering “laws of nature” to observations of a physical object and its states, while the distinct notion of “measurable” replaces the vague notion “observable”. On the basis of this notion, the roles of physical models and measurements within physics, different kinds of work, experiments, and laws are discussed. Next follows a discussion of different levels of change in science, after which this distinction is compared to Kuhn’s model. Finally, I present a new combination of “realism” and “constructivism”, which differs from both the “constructive empiricism” of van Fraassen and from different “empirical realisms”.