Wuliheron, this is typical misunderstanding of Einstein. What Einstein proposed has nothing to do with philosophy, but with mathematics, specifically with mathematical symmetries.
He proposed that:
a) nature could be invariant versus continuous shifts in velocity (any observed quantity f is same in any moving reference system: f(v) = f'(v'). This symmetry (=nothing in lab depends on lab's velocity) is the only postulate from which all Special Relativity mathematically follows as a corollary.
b) there is no difference between inertial mass m and gravitational mass m. General Relativity mathematically follows from this his assumption just as a corollary.
