's theory
BL 9 15 is one of the 20 principles of Fisher's theory, which is one of the ways in which he aimed to explain how markets operate. It is based on the idea that market prices are driven by the expectations of traders, and more specifically, that prices will eventually converge towards the expected value of a security over time. This is because, over time, the information available to traders will become more complete, allowing them to better price in their expectations and thus, the price will be stable at the expected value.