 Let us now analyse Heisenberg’s argument in more detail. Note that, even in this improved version, Heisenberg’s argument is incomplete. According to Heisenberg’s “measurement=meaning principle”, one must also specify, in the given context, what the meaning is of the phrase “momentum of the electron”, in order to make sense of the claim that this momentum is changed by the position measurement. A solution to this problem can again be found in the Chicago lectures (Heisenberg 1930: 15). Here, he assumes that initially the momentum of the electron is precisely known, e.g., it has been measured in a previous experiment with an inaccuracy \(\delta p_{i}\), which may be arbitrarily small. Then, its position is measured with inaccuracy \(\delta q\), and after this, its final momentum is measured with an inaccuracy \(\delta p_{f}\). All three measurements can be performed with arbitrary precision. Thus, the three quantities \(\delta p_{i}, \delta q\), and \(\delta p_{f}\) can be made as small as one wishes. If we assume further that the initial momentum has not changed until the position measurement, we can speak of a definite momentum until the time of the position measurement. Moreover we can give operational meaning to the idea that the momentum is changed during the position measurement: the outcome of the second momentum measurement (say \(p_{f}\) will generally differ from the initial value \(p_{i}\). In fact, one can also show that this change is discontinuous, by varying the time between the three measurements.