The preceding chapters showed that the measurement theory of current particle physics has two predominant features. First, it is heterogeneous. To put it in Nancy Cartwright's terms, it has a piecemeal structure.¹ It combines non-relativistic and relativistic laws from classical physics and several quantum theories. Second, it is based on powerful unifying principles. The construction of the length, time, and mass scales establishes at least a semantic unity of physics. The use of such a heterogeneous measurement theory is no grist for Nancy Cartwright's mills. It is based on trust in a hidden unity of physics. In this chapter I will investigate the heuristic assumptions and the unifying principles behind this trust.