Spaced repetition
Spaced repetition is a learning technique that schedules reviews at expanding intervals to exploit the spacing effect and fight Ebbinghaus’s forgetting curve — “newly introduced and more difficult flashcards are shown more frequently, while older and less difficult … less frequently.” It’s the memory-internalizing branch of tools-for-thought the wiki was missing: where the PKM lineage (zettelkasten, memex, notion…) stores knowledge externally, spaced repetition moves it into the head. Source: Wikipedia.
Software lineage
Algorithmic schedulers automate the intervals: SuperMemo (the SM-0…SM-18 family), Anki (open-source; now the FSRS scheduler in 23.10+). The user rates recall; the system times the next review. This is a tool-for-thought in the strict sense — software augmenting a cognitive faculty (memory), kin to how knowledge-graphs augment association.
Place in the lineage
Spaced repetition completes a complementary pair with the wiki’s note-taking pillar: externalize (Zettelkasten/obsidian/roam-research) vs internalize (spaced repetition). Both are Engelbart-style augmentation — and the modern “spaced-repetition memory systems” framing treats the two as one practice (turn notes you want to keep into cards you retain). A bridge, too, to llm-wiki: an external compounding store vs. internal compounding recall.
Related
tools-for-thought · zettelkasten · memex · augmenting-human-intellect · obsidian