(See Causality (physics).) Unlike causality, the concept of meaning is purely subjective except in symbol systems such as language/grammar and mathematics – both systems which depend on prior agreement on the arbitrary assignment of specific meanings to specific symbols or symbol groups.. The outlier effect can be used to explain events that are unlikely to occur by chance. Littlewood's Law states that individuals can statistically expect a significant event to happen for them at a frequency of about once per month. Littlewood assumes a significant event frequency at one in a million with a duration of one second, and that an individual is alert to significant events for eight hours per day. The 1993 book "The Celestine Prophecy" by James Redfield encouraged its readers to seek meaning from grouped events. From the book's First Insight: "In each of our lives occurs mysterious coincidences – sudden, synchronistic events that, once interpreted, lead us into our true destiny'. The book was a 1996 best seller, spent 165 weeks on "The New York Times" Bestseller list, and has now sold over 20 million copies.