“We learn, in part, by forgetting.” —Robert A. Bjork
Spaced repetition is the practice of distributing study across increasing intervals of time rather than concentrating it into a single sitting. It is the load-bearing principle behind every serious flashcard system—see the companion Anki Explained essay for how the software puts it to work—and it is the most counter-intuitive of the three pillars of durable learning, because it deliberately makes each study session feel harder. These notes work from the empirical phenomenon (the spacing effect), through the theory of why it holds, to the mathematics and the scheduling algorithms that operationalise it.
