physcis as an ab initio model

Why do we call quantum physics an ab initio model? Why do we call classical mechanics an ab initio model? Is classical electromagnetism an an ab initio model (and if it is one, what are its first principles)?

All these fields are based on an ab initio model because they are allo based on a set of priciples and equations that can be used to explain the phenomena they describe. For quantum we have Schördinger equations, classical mechanics have Newtown laws, electromagnetism have Maxwell equations.