As builders of applications and systems we commonly utilize metaphors to visually represent abstract concepts and information to users. One of the most commonly used metaphors for visually expressing hierarchical information is a tree. How did this particular metaphor become so prolific? Computer systems are not inherently structured as a tree metaphor, graph structures of vertices and edges are more natural, but are often displayed this way. If you haven’t familiarized yourself with semiotics, or the discipline of conveying meaning, I have written previously about it.
The most commonly used tree of knowledge metaphor is based on the Porphyrian tree. The Porphyrian tree originates with Porphyry of Tyre around 265 AD and his writing of Isagoge, an introduction to logic and philosophy. The Porphyrian tree, scala praedicamentalis (categorical scale) in latin, is a metaphorical tree of knowledge representing categories of information with dichotomous divisions. The root of the tree is called the summum genus (supreme genus) with its differentiae (branches). The tree was originally meant to reorganize Aristotle’s categorization system, one intended to encompass all human ideas, but throughout the medieval ages and renaissance became a standard for organizing logical taxonomies. For over 1500 years students read Isagoge resulting in deep and lasting adoption of the concept for visually mapping taxonomies.
While the Porphyrian tree is used extensively in grammar, logic and biological structures, it is also heavily used in computer science. Filesystems are a common representation of a Porphyrian tree. As a consumer of the visual representation we understand information becomes more inclusive of content the higher in the structure with less fidelity in the information presented. It is this reason the metaphor is so commonly utilized, humans do not easily process a visual representation of a filesystem with every file depicted in one window with all relevant organizational information. It is the ability to traverse the tree structure and fold in branches that a simplier mental model is formed.
Porphyry did not illustrate his original tree stucture in Isagoge, here is one rendered by Finder.