Whenever we know something 100%, when we know something like the back of our hand we say I know it by heart. When we know something by heart it means we repeated it a lot in our heart. See we repeat it in the head not in the heart but why do we use the heart as the epicenter of the expression. The process of repeating in your head is called learning by rote. Rote is from the latin word Rota and it means wheel. It is an analogy of repeating something in our heart many times to learn it, like a wheel that keeps rolling. The Greeks thought that it was from the heart we think and not the brain.
Did you know that the Greek language used in current day Greece is completely different from the ones used in the Forums of Athens millenniums ago by the Ancient people.