This book seeks to answer the question, What is holiness? What do we talk about when we talk about holiness? We might describe many things as holy, but as Socrates says, what is the essential aspect, by which all holy acts are holy? Sanctum Sanctorum gives an account of the holy from within the Christian participatory tradition, and argues that holiness is included in a special category of divine names that Christian metaphysics calls transcendentals (which are five: being, one, truth, goodness, and beauty). Moreover, holiness stands in a hierarchical relationship to the other five transcendentals, as the culmination or concentration of the rest. Only by understanding holiness as the head of the transcendentals, as the transcendental, can one account for all the complexity the idea of the holy conjures. Therefore, holiness is the transcendental of the transcendentals. It adds the aspect of reverence to existence and, as such, it is constituted by the formula sanctum sanctorum (Holy-of-holies) which extends from the divine nature through the triune life to all creation.