The Spanish philosopher Sebastian Izquierdo was a luminary of seventeenth-century scholasticism. In his main philosophical work, Pharus scientiarum, Izquierdo claims to be the first to provide a systematic analysis of the notion of priority. Izquierdo is not the first to attempt an analysis of priority, but his treatment is original and rigorous. Izquierdo distinguishes between accidental and essential priority. This distinction crosscuts his distinction between relative and absolute priority. Izquierdo also recognizes four types of absolute priority: priority of duration, priority of origin, priority of nonmutual connection, and priority of worth. In this article I explain Izquierdo's various priority relations, their formal properties, and how they all cohere in a general theory of priority.