Hortensius (Latin: [hɔrˈtẽːsi.ʊs]) or On Philosophy is a lost dialogue written by Marcus Tullius Cicero in the year 45 BC. The dialogue—which is named after Cicero's friendly rival and associate, the speaker and politician Quintus Hortensius Hortalus—took the form of a protreptic. In the work, Cicero, Hortensius, Quintus Lutatius Catulus, and Lucius Licinius Lucullus discuss the best use of one's leisure time. At the conclusion of the work, Cicero argues that the pursuit of philosophy is the most important endeavor.