1. A memory system, comprising: a non-volatile memory device that stores user data and state information regarding the user data; and a memory controller that accesses the state information and the user data in a read operation of the non-volatile memory device, calculates a priori probabilities of the user data based on the state information, calculates a posteriori probabilities of the user data based on the a priori probabilities, and performs a soft-decision operation to detect logical states of the user data based on the a posteriori probabilities.