ab anitio

Quantum mechanics is considered a true ab initio model because it builds directly on fundamental physical postulates—such as the path integral formalism, where every possible path contributes to the probability amplitude—without relying on empirical fitting. In contrast, classical mechanics and classical electromagnetism are not strictly ab initio in the fundamental sense because they are emergent approximations and limiting cases of more fundamental underlying theories (quantum mechanics and quantum electrodynamics). While they can function as “first-principles” frameworks within their own macroscopic domains (starting from axioms like Newton’s laws or Maxwell’s equations), they ultimately break down at microscopic scales where the true quantum rules govern nature.