Isagoge

The work includes the highly influential hierarchical classification of genera and species from substance in general down to individuals, known as the Tree of Porphyry, and an introduction which mentions the problem of universals.

With the Arabicized name Isāghūjī, it long remained the standard introductory logic text in the Muslim world and influenced the study of theology, philosophy, grammar, and jurisprudence.

praedicabilis, that which may be stated or affirmed, sometimes called quinque voces or five words) is, in scholastic logic, a term applied to a classification of the possible relations in which a predicate may stand to its subject.

The scholastic classification, obtained from Boëthius's version of the Isagoge, modified Aristotle's by substituting differentia (diaphora) and species (eidos) for definition (horos).

Porphyry writes 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.