I have been thinking about Einstein's equivalence principle, wherein acceleration is equated with gravity. The point is: that this is just sleight of hand. Newton's whole theory of gravity is based on the fact of inertial mass. Without this concept it would be impossible to calculate the orbits of planets or the trajectory of comets. There would literally be no way to calculate gravitational effects. It is central to Newton's theory of gravity. The question is this: where did Einstein get off rephrasing this idea and what did he hope to gain from it if not to discredit Newton's theory of gravity? Surely, a simple ploy such as equating gravity and acceleration, creates enough doubt to let the thin edge of general relativity in?