Keith,

I am surprised that someone who advertises themselves as a private tutor in Maths and Science should say that. Proof is a concept in mathematical logic. Science advances by the formulation of hypotheses which can be falsified. Newton's theory of gravity is such a hypothesis, it can be falsified in extreme conditions, such as high accelerations (relativity) and maybe low accelerations (MOND). But there is, and can never be, an absolute proof, and nor is one required.