I think a lot of them still rely on intuition and human level thoughts experiment. See The Feynman Lectures on Physics for a physicist using intuition. For example, when he explains the Fermat Principe with the lifeguard problem: the path of least time running on the beach and swimming to the victim is not the least distance. If you consider that you run faster than you swim, then you should run a bit further so you swim a bit less. This is analogous to the bending of a light ray (refraction, “broken” stick) when it enters water. Image