This book is the result of almost ten years of research. It has accompanied me through an intense period of my life, from the end of my undergraduate studies in Rome across the years of my Ph.D. in London until my current Research Fellowship at Girton College (University of Cambridge, UK). I have grown with it, and with it, I have come to develop my philosophical ideas. Looking back, I can see the way they have evolved and focussed; how they came to be refined, and sometimes revised. I owe intellectual debts to many people who in various ways have contributed to the development of my ideas over this span.

My original intention of studying the exclusion principle dates back to 1996. At that time I was an undergraduate student in Rome, very keen on philosophy of science and history of modern physics. Reading Pauli's scientific correspondence, I was struck by a passage of a letter to Landé in which the famous exclusion principle was introduced as an ‘extremely natural rule’. It may have appeared ‘extremely natural’ to Pauli, but to me the overall manoeuvre seemed mysterious and intriguing. I could not help plunging into the details of this fascinating historical episode. I owe an old debt to my teachers Silvano Tagliagambe, who hooked me on philosophy of science, and Sandro Petruccioli, who encouraged me to consider Wolfgang Pauli as a possible research topic.

During the stimulating years of my Ph.D. at the London School of Economics, my research project received a new twist.