This volume sets out to explore the relationship between individual will (voluntas) and the legal rule. What unfolds in the following pages is a wide-ranging itinerary, moving between past and present, most notably ancient Rome and the contemporary world. The guiding question is as radical as it is enduring: in what way can voluntas (a psychological impulse internal to the individual) come to determine the legal rule? European private law tradition rests on the premise that legally binding acts - contract and will, to mention only two paradigmatic cases - derive their force from individual will. From the Roman sources arises, with exemplary force, the notion of lex privata: the idea that private will itself may generate binding legal norms. Such a premise immediately leads to further questions. Above all, it compels reflection on the authenticity of that will: what if voluntas is compromised? The law of defects (error, dolus, metus) opens the problem of whether distorted or corrupted will can truly sustain the validity and effects of a legal rule. The reflections gathered in this book approach the European civil law tradition as a broad and unified phenomenon, one in which law is inseparably bound to the historical and cultural contexts in which it takes shape.