In the course of a study of elementary particles, an analysis is given of a fundamental presupposition of many research programs, namely the belief in the ultimate unity of physics. It is argued tht this unity-idea is incorrect. By classical physics, relativity theory and quantum theory three distinct structures of nature are revealed. Next, the essential aspect of measurement, that a measurement always results in a record, is analysed. Recording implies irreversibility and entropy production. In modern elementary particle physics the (...) record itself is all important, and therefore the entropy production due to measurement is essential. It is argued that this aspect of measurement will play a central role in a distinct theory concerning fundamental characteristics of elementary particles, revealing a fourth structure of physical reality. Some basic notions of this new theory are proposed. Finally the meaning of irreversibility on the micro-level of elementary particles is discussed, in connection with the concept of time. (shrink)