To borrow an example from another science, and an "exact" one at that. The critics of the Marxian law of value are exactly in the same situation as would be the critic of the law of gravity, who would declare that law to be false for the reason that bodies do not fall in actual experience in accordance with the rules formulated by it. Indeed, such a critic would be in a better position than the Marx-critics. For, while according to the laws of gravity falling bodies acquire an acceleration of 981 centimeters per second, and that irrespective of their nature, form or size, the "facts of experience" prove conclusively that not one body in a million actually falls at that rate, and any child of some intelligence will tell you that the nature, the form, and the size of a falling object, make all the world of difference in the velocity which it can acquire. Yet, the law of gravity is correct when properly understood. And the Marxian law of value is no less correct. But it requires a greater intelligence than that usually displayed by intelligent children, observers of "facts of experience," and some Marx-critics, to understand it properly. Therein lies the whole trouble.
