The Tree of Porphyry (also known as scala praedicamentalis) is a classic device for illustrating what is also called a "scale of being". It was suggested—if not first, then most famously in the European philosophical tradition—by the 3rd century CE Greek neoplatonist philosopher and logician Porphyry.
Porphyry suggests the Porphyrian tree in his introduction (in Greek, "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. But, diagrams were eventually made, and became associated with the scheme that Porphyry describes, following Aristotle.
Porphyry's Isagoge was originally written in Greek, but was translated into Latin in the early 6th century CE by Boethius. Boethius's translation became the standard philosophical logic textbook in the Middle Ages. Until the late 19th century, theories of categories based on Porphyry's work were still being taught to students of logic.
The following passage by philosopher James Franklin gives some hint as to the history of the Porphyrian tree:
- In medieval education, the standard introduction to Aristotle's works was via Porphyry's Isagoge, and division entered the educated consciousness in the form of 'Porphyry's Tree'. It is not clear that Porphyry himself, in the relevant passage, went any further than Aristotle in recommending division. But his brief comment was developed into the Tree by medieval logicians. It appears in William of Sherwood's Introduction to Logic and is given the name arbor Porphyrii in the most popular medieval logic, Peter of Spain's Summulae Logicales. Linnaeus's system of static and discrete species was simply the result of filling in the abstract Tree with the names of actual species.
Thus, the notion of the Porphyrian tree as an actual diagram comes later than Porphyry himself. Still, scholars do speak of Porphyry's tree as in the Isagoge and they mean by this only that the idea of dividing genera into species via differentiae is found in the Isagoge. But, of course, Porphyry was only following what was already in Aristotle, and Aristotle was following what was already in his teacher, Plato.