In 1885, Hermann Ebbinghaus systematically measured his own memory. He memorised lists of nonsense syllables, then tested his recall at intervals. The results produced the forgetting curve: without reinforcement, 50% of new information is forgotten within an hour, 70% within a day, 80% within a week.
The forgetting curve is not a failure of attention or effort. It is how the human memory system works by design — information that is not retrieved decays. The mechanism for reversing decay is retrieval: actively recalling information strengthens the neural pathways that store it. Passive re-reading does not strengthen those pathways; it only provides the information again without exercising the recall mechanism.
The practical implication: reading this book once is insufficient. A revision system built into your workflow is necessary. This chapter gives that system.