The ancient Greeks thought that the heart was where intelligence and memory was stored in a human body. Nowadays, it is known by science that this is not true - this is a task for the brain. However, the idiom "know something by heart" still carries this old Greek concept, and can be found in several languages throughout the Europe. Used in this sense, it implies that the person knows something by their "soul"; or that they are deeply connected to the subject rather than knowing by completely methodical and scientifical means.