 Finally, on a more formal level, we note that Bohr’s derivation does not rely on the commutation relations  (1)  and  (5),  but on Fourier analysis. These two approaches are equivalent as far as the relationship between position and momentum is concerned, but this is not so for time and energy since most physical systems do not have a time operator. Indeed, in his discussion with Einstein (Bohr 1949), Bohr considered time as a simple classical variable. This even holds for his famous discussion of the “clock-in-the-box” thought-experiment where the time, as defined by the clock in the box, is treated from the point of view of classical general relativity. Thus, in an approach based on commutation relations, the position-momentum and time-energy uncertainty relations are not on equal footing, which is contrary to Bohr’s approach in terms of Fourier analysis. For more details see (Hilgevoord 1996 and 1998).