Quantum physics is based on its fundamental laws

Ab initio means “from first principles.” Quantum mechanics is called an ab initio theory because it predicts physical behavior directly from fundamental laws without empirical fitting. Classical mechanics and classical electromagnetism are also ab initio within their valid domains, since they are based on universal first principles (Newton’s laws and Maxwell’s equations). However, they are effective, emergent limits of quantum theory, which is why ab initio is most commonly associated with quantum-based models today.