Quantum physics, classical mechanics and classical electromagnetism are all examples of ab initio models. These respectively have the Schrodingers equation, Newton’s laws and Maxwell’s Laws as first principle. From these first principles everything else that’s important in the model can be derived. This is why they are ab initio models. I hope one day we’ll have a universal theory of everything containing a lot of ab initio models, but untill then, the physics we have now will have to do.