Juan Huarte de San Juan (1529–1588) was a Spanish physician and natural philosopher, whose Examen de ingenios para las ciencias (1575) was a landmark contribution to early modern thought on natural disposition and professional guidance. His treatise centers on the concept of ingenium, an individual’s natural disposition and/or abilities, and proposes an ideal for social reform based on aligning each individual’s ingenium with the profession best suited to them. Huarte’s Examen was recognized for its practical relevance and achieved wide circulation and translation across Europe, though it also attracted the attention of the Inquisition and was subject to censorship. In modern times, the work has been studied across disciplines for its historical and intellectual significance. Within psychology, in particular, Huarte’s emphasis on the physiological basis of intellectual differences has led some scholars to view him as a precursor of assessment and organizational psychology.