---------------------------------------- S15d. Foundations, theory and experiment ---------------------------------------- Foundations of physics is the quest for getting the mathematical concepts right to be able to do correct physics and think correctly about it. Without correct concepts operational statements have no meaning. The theory defines what a measurement is. Outside the immediate realm of everyday experience, one needs already the conceptual basis to even discuss what has operational meaning. These statements apply both to good and bad theories. Even a bad theory defines what a measurement is; it just defines is more poorly. There is in fact a crossfertilization between measurement and foundations. If one gets better the other profits from it. On the other hand, fuzzy foundations lead to poor judgment and ambiguity in measurements, and poor measurements lead to low discrimination among theoretical alternatives. One can observe from history that progress in concepts lead to better inverstigations of nature, and better experiments lead to higher demands on the theory, forcing people to look for more stringent concepts and simpler or more encompassing frameworks.