“… the beauty of tagging is that it taps into an existing cognitive process without adding add much cognitive cost. At the cognitive level, people already make local, conceptual observations. Tagging decouples these conceptual observations from concerns about the overall categorical scheme. The challenge for tagging systems is to then do what the brain does – intelligent computation to make sense of these local observations, and an efficient, predictable way to ensure findability.”
Read Rashmi Sinha’s article, A cognitive analysis of tagging