This chapter is the first English translation (by E. Crull and G. Bacciagaluppi) of Grete Hermann's fundamental essay of 1935 on the foundations of quantum mechanics. This essay, written after a protracted visit to Heisenberg's research group in Lepizig, is possibly the most remarkable and complete early philosophical analysis of quantum mechanics. After criticising known arguments for the completeness of quantum mechanics, including von Neumann's proof, Hermann argues that the notions of causality and predictability have to be separated, and that only the latter but not the former is lost in quantum mechanics. Indeed, she argues using the Heisenberg microscope as her prime example, that causal chains can be reconstructed after the fact for any quantum mechanical effect, so that quantum mechanics is in fact already causally complete. She goes on to analyse Bohr's views on complementarity, and to sketch the natural-philosophical picture provided by quantum mechanics along the lines of a neo-Kantian (neo-Friesian) transcendental idealism .