What is Commonplace Book?
A commonplace book is a personal notebook where a reader collects quotations, ideas, and observations from their reading, usually organized by theme rather than by date. Kept since antiquity and especially popular during the Renaissance, it is the ancestor of the modern highlight library and second brain.
The name comes from the Latin locus communis, a common place, meaning a general theme under which excerpts could be filed. Unlike a diary, which is chronological and introspective, a commonplace book is thematic and outward-facing: it gathers the best of what a reader encountered so it can be found again and reused in writing, speech, and argument. Figures from Milton to Thomas Jefferson kept them, and Renaissance educators taught commonplacing as a formal method.
The practice encodes durable learning principles. Copying a passage by hand forces selection, the judgment of what deserves keeping, and filing it under a theme forces categorization, an act of elaboration that links the new excerpt to an existing structure of ideas. Rereading a themed collection then juxtaposes authors who never met, which is where original connections tend to appear.
Digital highlighting recreates commonplacing at web scale: highlights are the excerpts, tags are the commonplace headings, and search replaces the index. What the tradition adds to the tools is the habit of returning, since a commonplace book was meant to be read as much as written.