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The Isagoge or "Introduction" to Aristotle's Categories (text) was a the standard textbook on logic for more than a thousand years after his death in the third century, and was one of the six books of the Ars Vetus or corpus of writings on Aristotelian logic that survived in the Latin West during the dark ages and early middle ages, before Aristotle's other books were recovered. It was composed by Porphyry in Greek in Sicily during the years 268-270, and sent to Chrysaorium, according to the commentators Ammonius, Elias, and David.

It includes an influential hierarchical classification ('the Tree of Porphyry') of genera and species from the most general genus down to the most specific species and individuals, and an introduction which mentions the problem of universals. Boethius' translation of the work, in Latin, became a standard medieval textbook in European schools and universities. Many writers, such as Boethius himself, Averroes, Abelard, Scotus, wrote commentaries on the book. Other writers such as Albert of Saxony and William of Ockham incorporated them into their textbooks on logic.

The earliest Latin translation, no longer extant, was made by Marius Victorinus in the fourth century. Boethius relied upon it in his own translation. The earliest known Syriac translation was made in the seventh century by Athanasius of Balad. There is also an early Armenian translation. The Introduction was translated into Arabic by Ibn al-Muqaffa‘ from a Syriac version. With the Arabicized name Isaghuji it was for a long time a standard introductory logic text in the Muslim world and influenced the study of theology, philosophy, grammar, and jurisprudence. The work contains on account of the 'predicables' (Lat. praedicabilis, that which may be stated or affirmed), also known as the quinque voces. The predicables are a classification of the possible relations in which a predicate may stand to its subject, based on the original classification given by Aristotle in the Topics (iv. 101 b 17-25): definition (horos), genus (genos), differentia (diaphora), property (idion), accident (sumbebekos). In the scholastic classification, species (eidos) is substituted for definition.

The work is celebrated for prompting the medieval debate over the status of universals. Porphyry writes

For the moment, I shall naturally decline to say, concerning genera and species, whether they subsist, whether they are bare, pure isolated conceptions, whether, if subsistent, they are corporeal or incorporeal, or whether they are separated from or in sensible objects, and other related matters. This sort of problem is of the very deepest, and requires more extensive investigation.

Though he did not mention the problem further, his formulation constitutes the most influential part of his work, since it was these questions that formed the basis of medieval debates about the status of universals. Do universals exist in the mind, or in reality? If in reality, are they physical things, or not? If physical, do they have a separate existence from physical bodies, or are they part of them?

Logic Museum online text


  • A. Busse (ed): Porphyrii Isagoge, Commentaria in Aristotelem Graeca IV I (Berlin, 1887)

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