I have studied physics for about three years at university but after these years I still don't know how Newton derived his laws.
I know that he tried to find the best model to describe the motions but It's a bit hard to believe for example how did he knew that an object can move at constant velocity without an external force? how he could be sure about that?!
Or, how could he derive his third law?, there was no instrument to measure forces or velocities..., he couldn't lie on the observations without any reliable data. Should I consider his laws as principles?!