 There is a second notable difference between  (2)  and  (9).  Heisenberg did not give a general definition for the “uncertainties” \(\delta p\) and \(\delta q\). The most definite remark he made about them was that they could be taken as “something like the mean error”. In the discussions of thought experiments, he and Bohr would always quantify uncertainties on a case-to-case basis by choosing some parameters which happened to be relevant to the experiment at hand. By contrast, the inequalities  (9)  and  (12)  employ a single specific expression as a measure for “uncertainty”: the standard deviation. At the time, this choice was not unnatural, given that this expression is well-known and widely used in error theory and the description of statistical fluctuations. However, there was very little or no discussion of whether this choice was appropriate for a general formulation of the uncertainty relations. A standard deviation reflects the spread or expected fluctuations in a series of measurements of an observable in a given state. It is not at all easy to connect this idea with the concept of the “inaccuracy” of a measurement, such as the resolving power of a microscope. In fact, even though Heisenberg had taken Kennard’s inequality as the precise formulation of the uncertainty relation, he and Bohr never relied on standard deviations in their many discussions of thought experiments, and indeed, it has been shown (Uffink and Hilgevoord 1985; Hilgevoord and Uffink 1988) that these discussions cannot be framed in terms of standard deviations.