Quantum physics, classical mechanics, and classical electromagnetism can all be considered ‘ab initio models’, as they reduce the systems they observe into the simplest model possible.

In order to represent a system which obeys the laws of classical mechanics, no additional variables or equations need to be added to the existing equations belonging to classical mechanics. Similarly, if one was to remove an equation/variable from classical mechanics, our model would be incomplete.

This applies to quantum physics and electromagnetism as well, electromagnetism’s first principles being Maxwell’s equations.