You have to learn a poem to recite in class. How do you learn it? By heart. You’re an actor and you have to memorize your lines. You learn them by heart. You have to give a big speech. Better learn by heart!
It is our brains which house memories and yet if you actually said “I need to learn this material by brain,” it would sound extremely odd to the ear of an English speaker. In fact, it would sound incorrect.
‘By Heart’ Is an Idiom
When we talk about learning something by heart, we are using an idiom. Idioms, as you can see by this explanation, are not literal, but figurative. To learn something by heart means the same as learning something ‘by rote,’ itself an idiom.
Both of these idioms mean to commit something to memory verbatim. By rote has a slightly different origin even though it is today used as a synonym. Originally, to learn something by rote was to have learned to sing or chant it to the accompaniment of a harp. Another word for harp was rote.
We Once Believed the Heart Housed our Memories and Thoughts
But, why do we say heart instead of brain or mind? Because, at one time, humans believed that thoughts were housed in the heart. In fact, they believed that the heart was the center of thoughts, emotions, and even the human soul. The brain was not understood to have any function whatsoever. We can see this in the way Egyptians treated mummification. The heart was carefully preserved because the deceased would need it in the afterlife. The brain was removed and discarded.
Both these idioms are informal. To ‘commit something to memory’ is much more formal, while ‘to memorize’ is used in both formal and informal English.
Learning by heart is also a good way to illustrate how an idiom that sounds correct in one language may sound incorrect in another. To a Chinese person, ‘to learn by heart’ would sound only partially correct, for example. This is because the Chinese do not see the heart and the mind as dual concepts. To learn something by heart would most likely be interpreted as an incorrect way of saying “put your heart into it” or “use your heart to learn something,” meaning to work and study hard.