The article systematizes a process of a physical cognition (phases: an experiment, a hypothesis, a law of physics, a principle, a physical theory) and concentrates on general principles on which the physics is based. It deals with variational principles of physics – the principle of least action in classical mechanics, in Maxwell´s theory of electromagnetic field and optics. It follows up with an essay about principles of conservation in classical physics, points at their connection with a symmetry in nature and formulates the theorem of Noether and from it resulting principles of conservations of energy, of momentum and angular momentum. The final parts are dedicated to the principles of thermodynamics and general principles and principles of conservation in relativistic and quantum physics.