Isagoge in the context of "William of Ockham"

Porphyry (/ˈpɔːrfɪri/; Koine Greek: Πορφύριος, romanized: Porphýrios; c. 234 – c. AD 305) was a Phoenician Neoplatonic philosopher born in Tyre, Roman Phoenicia during Roman rule. He edited and published the Enneads, the only collection of the work of Plotinus, his teacher.

He wrote original works in the Greek language on a wide variety of topics, ranging from music theory to Homer to vegetarianism. His Isagoge or Introduction, an introduction to logic and philosophy, was the standard textbook on logic throughout the Middle Ages in its Latin and Arabic translations. Porphyry was, and still is, also well known for his anti-Christian polemics. Through works such as Philosophy from Oracles and Against the Christians (which was banned by Constantine the Great), he was involved in a controversy with early Christians.

In philosophy (particularly the theory of categories), the Porphyrian tree (also spelled Porphyrean tree) or Tree of Porphyry is a classic device for illustrating a "scale of being" (Latin: scala praedicamentalis), attributed to the 3rd-century CE Greek neoplatonist philosopher and logician Porphyry, and revived through the translations of Boethius.

Porphyry suggests the tree in his introduction ("Isagoge") to Aristotle's Categories. Porphyry presented Aristotle's classification of categories in a way that was later adopted into tree-like diagrams of two-way divisions, which indicate that a species is defined by a genus and a differentia and that this logical process continues until the lowest species is reached, which can no longer be so defined. No illustrations or diagrams occur in editions of Porphyry's original work; diagrams were eventually made, and became associated with the scheme that Porphyry describes, following Aristotle.

In logic and formal semantics, term logic, also known as traditional logic, syllogistic logic or Aristotelian logic, is a loose name for an approach to formal logic that began with Aristotle and was developed further in ancient history mostly by his followers, the Peripatetics. It was revived after the third century CE by Porphyry's Isagoge.

Term logic revived in medieval times, first in Islamic logic by Alpharabius in the tenth century, and later in Christian Europe in the twelfth century with the advent of new logic, remaining dominant until the advent of predicate logic in the late nineteenth century.