Critical human interests are affected on a daily basis by appeal to past decisions deemed to be `legally valid'. They include statutes, deportation orders, judgments, mortgage contracts, patents and wills. Through the technique of validity, lawyerly reasoning settles morally pressing matters in a way that largely bypasses moral argument. Legal philosophy has paid considerable attention to validity criteria, but it has neglected to explore validity's point: whether, and if so how, the pervasive technique of validity can contribute to a legal system's ability to realise justice and human rights. This book shows that validity can help a political community to foster justice precisely because validity does not primarily turn on moral considerations. Validity serves to both allocate, and limit, a distinct kind of power, a power that is key to forging valuable forms of enterprise and commitment in pursuit of individual and collective self-direction. By entrusting the capacity to decide to those who, in justice, ought to bear it, validity can enable persons and institutions to rally the resources and opportunities that only large-scale behavioural convergence can afford, thereby weaving a fabric of just relationships within the systemic framework of law.