This paper examines the transformation which occurs in Heisenberg's understanding of indeterminacy in quantum mechanics between 1926 and 1928. After his initial but unsuccessful attempt to construct new quantum concepts of space and time, in 1927 Heisenberg presented an operational definition of concepts such as ‘position’ and ‘velocity’. Yet, after discussions with Bohr, he came to the realisation that classical concepts such as position and momentum are indispensable in quantum mechanics in spite of their limited applicability. This transformation in Heisenberg's thought, which centres on his theory of meaning, marks the critical turning point in his interpretation of quantum mechanics.

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Author notes

Kristian Camilleri is a lecturer at the University of Melbourne in the History and Philosophy of Science department. He studied physics and History and Philosophy of Science at the University of Melbourne before completing his PhD on Heisenberg and Quantum Mechanics: the Evolution of Philosophy of Nature in 2005. His current research focuses on the historical foundations of modern physics.