Heisenberg discusses the relationship between the experimental results and the theoretical construction of quantum physics and then to its epistemological and ontological assumptions. The investigation is carried out both from a historical and a theoretical point of view. The main ideas of quantum physics are presented through a comparison with both classical physics and the development, from early Greek philosophy to modern rationalism, of the philosophical quest for a fundamental principle of reality or structure of matter. Special attention is paid to the role played by the notions of ‘probability’ and ‘indeterminacy’ as the central notion of the ‘new’ physical universe, changing dramatically the perception that man has of the status of its reality and of his own place in it.