Around 520 AD, the Roman philosopher Anicius Manlius Severinus Boethius was expounding on Greek logic. To assist his students, he drew a diagram that later came to be known as the arbor porphyriana or arbor porphyrii. It is a branching diagram, starting at its top, which demonstrates a simple system of classification based on the teaching of Aristotle, proceeding from the general to the specific.
While Boethius' own personal manuscript does not survive, there is nothing to suggest that the diagram, as drawn in later copies of it, was a mere embellishment by copyists. In fact, Boethius explicitly invites his reader to look at the visual figure that accompanies his text (...rei subjecta descriptio sub oculis ponat exemplum) and then refers to it as the "above" diagram.
This stemma is an integral part of early extant manuscripts and is published in the flow of the text of both the critical and library editions [*]Brandt, Samuel (ed.). Corpus Scriptorum Ecclesiasticorum Latinorum 48. Leipzig, 1906: Editio secunda, III.3, page 209 uses a purely typographical presentation: the bare words, connected by one column of vertical strokes and a stack of horizontally arranged curly braces. Each such accolade horizontale has its nose turned up and its twin tails turned down. [*]In Migne, Patrologia Latina 64, 103D, the stemma is a presented in a black and white engraving with foliate decoration, apparently based on one of the medieval manuscripts. of this book, In Isagogen Porphyrii Commentum. The arbor porphyriana is striking because it is more elaborate than a simple cladogram. It is in fact two parent-child stemma diagrams overlaid on one another.
One of these diagrams, the central axis, is a hierarchy of description, showing increasingly specific terms. Boethius' six-node prototype describes an individual human being with growing specificity as being an instance of reality, body or corporealness, organisms, animals, intelligent life and humans. The remainders of each of these divisions, of example the beasts as non-intelligent life, are not present, merely implied. Before reading on, I suggest you look at my reconstruction (link) where these implied remainders are shown with dotted lines around them.
The diagram overlaid on this axis, constituted by the two outer columns, contains what might be called an argument "tree", with a series of branching tests of the differentiae. The true results are then passed to the central column as properties of the next-named genus or species, the false results end in the void on the right and the function repeats.[*]If this were a computer script, and the comparison is not as far-fetched as it might seem, one would describe this progress as a looping function which returns on true and loads the next value in sequence, while a false result ends.. In the Boethius prototype, the five node pairs are: corporeal or incorporeal, animate or inanimate, aware or non-aware, rational or non-rational, and mortal or immortal.
Five specimens of mankind (Plato, Virgilius, Socrates, Cicero, Cato) are scattered at the foot of the original manuscript diagram as examples to help the learner, but have not been included in the example as presented on this website.
This complexity implies that a literate person in late antiquity could readily understand how to comprehend text arranged in a simple, top-to-bottom cladogram or flow chart, and would doubtless have found a more sophisticated visual aid such as that composed by Boethius intellectually stimulating, yet not impossible to understand.
Over time, what we would nowadays call a text node came to be initially described as a clipeus, the Latin term for a shield, used figuratively in the sense of a label. Early medieval editions such as that in Cologne have no enclosures drawn around the clipeus, but in later editions of Boethius's diagram, these clipei are circumscribed by a roundel, or medallion. Elsewhere on this website we discuss whether or not the roundel was an early medieval fashion. Since it is the form familiar to us, I have used roundels in the reconstruction.
Boethius does not call the diagram a tree, but the term arbor came to be applied to it by its medieval users. Illustrators added a third layer to the motif: a tree as a background image, with the central axis descending via the tree-trunk while the nodes sprout foliage. Medieval educational practices stressed motifs as mnemonic aids. The modern term for the Boethius figure, arbor porphyriana, was not developed until 750 years after Boethius' death, in the writings of the medieval scholar Petrus Hispanus, and refers to the philosopher Porphyry, the subject of Boethius' teaching notes.
It is conceivable that Porphyry, who was born about 234 AD, devised the arbor porphyriana himself, but there is no recognized evidence for this traditional assumption, so it seems safest to give credit to Boethius as the draughtsman of the figure.
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