We call quantum physics an ab-initio model because it starts from the Schrödinger equation and all other properties of the system can (theoretically) be derived of it without the need for experiments. We call classical mechanics ab-initio because it functions according to the same principle: Newton’s laws allow you to deduce the behaviour of a classical system without having to do the experiment. Classical electromagnitism also is an ab-initio model with Maxwell’s equations serving as the fundamental equations.