 Since the above inequalities  (9)  and  (12)  have the virtue of being exact, in contrast to Heisenberg’s original semi-quantitative formulation, it is tempting to regard them as the exact counterpart of Heisenberg’s relations  (2)–(4).  Indeed, such was Heisenberg’s own view. In his Chicago Lectures (Heisenberg 1930: 15–19), he presented Kennard’s derivation of relation  (9)  and claimed that “this proof does not differ at all in mathematical content” from his semi-quantitative argument, the only difference being that now “the proof is carried through exactly”.