How do we know that the carbocation mechanism for addition of HX to alkenes is correct? The answer is that we don’t know it’s correct, or at least we don’t know with complete certainty. Although an incorrect reaction mechanism can be disproued by demonstrating that it doesn’t satisfactorily account for observed data, a correct reaction mechanism can never be entirely proven. The best we can do is to show that a proposed mechanism is consistent with all known facts. If enough facts are satisfactorily accounted for, then the mechanism is probably correct.