Here's a breakdown:
* Medieval belief: During the Middle Ages, people believed that the heart was the seat of emotions and intellect, not the brain. They believed that memories were stored in the heart, and reciting something "by heart" meant literally knowing it from the heart itself.
* Evolution of the idiom: Over time, the scientific understanding of the brain developed, and we learned that memories are stored there. However, the idiom "to know by heart" persisted because of its metaphorical meaning. It signifies a deep understanding and familiarity with something, as if it were part of your very being.
* Modern usage: Today, the idiom "to know by heart" signifies being able to recall something perfectly without having to think about it, as if it were imprinted on your mind. It's used for anything from memorized poems and speeches to lyrics of songs and important facts.
So, while the literal meaning of "knowing something by heart" is no longer valid, the metaphorical meaning continues to resonate with us, showcasing the powerful connection between memory and emotion.