The idea of effective theories is not new. In the 19th century and earlier, scientists developed continuum equations such as the Navier-Cauchy equations describing the behavior of isotropic elastic solids and the Navier-Stokes equations for incompressible viscous fluids. These equations were, and still are, remarkably safe. This means that once one inputs the appropriate values for a few phenomenological parameters (such as Young’s modulus and the sheer stress in the Navier-Cauchy equations), one arrives at equation models that allow us to build bridges and buildings that do not collapse. It is remarkable that a theory/model that almost entirely fails to refer to the details of the atomic and molecular structure of a steel beam, say, can be so successful and safe. A question of deep philosophical interest concerns how this can be the case. The phenomenological parameters must encode at least some details about the atomic and molecular make up of the beam. (Hence, the “almost” in the statement above.) 