Here are two sentences to try and memorize. The first one is English, the second one is Finnish.

“This is easy to memorize”

“Tämä on helppo muistaa”

That might have worked but if we make it longer the complexity adds up.

“This is easy to memorize. I know English so even if I don't get all the words correctly I can fill in the rest if need be.”

"Tämä on helppo muistaa. Osaan suomea, joten vaikka en saisi kaikkia sanoja oikein, voin tarvittaessa täyttää loput."

Not so easy. (Finnish is a weird language but anyway)

When we don’t understand the words the memorization becomes more reductive in a sense. We need to commit each word or even letter to memory. If we understand the words we get the big picture automatically. Then it becomes quite easy to reconstruct the individual words. Even if the exact words become slightly different, it is second nature for us to get the gist of it correctly.

This same principle applies to music. If a pianist wants to learn a piece of music by heart, the first goal should be to understand the musical construction rather than memorizing the notes.

Here is what this means in practice. This is the beginning of Chopin’s nocturne op 9 no 2:

There seems to be a lot going on here. For something that sounds quite simple, the sheet music seems a bit daunting.

If I were to imagine memorizing the left hand it might go something like this:

“Ok first we have an Eb flat in the base then we take the notes G and Eb flat simultaneously and then Bb flat Eb flat and G simultaneously. Alright then Eb flat again in the base this time an octave higher, then we have Ab flat and D simultaneously and then C flat, D and Ab flat simultaneously. Whew, that is just the first two beats of the first bar. “

Doing it that way becomes like memorizing Finnish (assuming you don’t know that language). Contrast this with another way of thinking about it. This Nocturne is a tune, a song. It has a melody and a harmony. Chopin made a particular arrangement with ornaments and a way of playing the chords to make it sound full and luscious on the piano. Let's learn the tune before we learn Chopin’s arrangement of the tune. It looks like this:

Much better. The harmonies can be memorized quite easily by simplifying the first bar as a tonic chord with a dominant on the second beat as a neighboring chord. The second bar is dominant leading to the minor chord on the second degree. The third bar uses a chromatic baseline surrounding the dominant.

When we know this by heart, we can do several things, including but not limited to playing Chopin’s arrangement of the tune. We could play it in a different key, come up with alternate colorings of the chord, add ornaments of our own to the melody, or invent a different accompaniment. This would be equivalent to knowing the plot of a story and telling it in our own words.

To sum up, memorizing a piece of music sets us up for a slightly wrong focus. The goal should be to understand the basic construction of the music, memorizing it being a byproduct.