Intellectual setting
Porphyry studied in Athens under the Platonist critic and rhetorician Cassius Longinus before moving to Rome in 263 CE to join Plotinus's circle, becoming his closest student and, after Plotinus's death around 270 CE, the editor of his teacher's scattered treatises. He wrote during the later third century's growing confrontation between traditional Greco-Roman philosophical culture and an increasingly assertive Christianity, to which Against the Christians was a direct response.
Key ideas
The Isagoge and the problem of universals
Porphyry's introduction to Aristotle's Categories posed three unanswered questions about genera and species - whether they exist in reality or only in thought, whether if real they are corporeal or incorporeal, and whether they exist separately from sensible things - that became the starting point of the medieval scholastic debate over universals once Boethius translated the work into Latin.
Against the Christians
In his largely lost treatise Against the Christians, Porphyry subjected Christian scripture to historical and philosophical criticism, notably arguing on internal evidence that the Book of Daniel was written during the Maccabean period rather than by a sixth-century prophet, a work later ordered destroyed by Christian emperors.
Legacy
Porphyry's Isagoge became a standard logic textbook throughout the Middle Ages, transmitted to the Latin West through Boethius's translation and to the Islamic world through Arabic commentators such as al-Farabi, and his edition of the Enneads preserved Plotinus's philosophy for later readers. Against the Christians provoked repeated imperial bans and book-burnings, and his source-critical argument about the dating of Daniel anticipated later historical-critical biblical scholarship.