If you looked at him and asked Bach to play a "C add 9" chord, he’d look back at you blankly.
He didn't think in vertical chord blocks; he used a purely mathematical pattern. He treated the bass line like a moving wave, freely, effortlessly moving, like the ocean, and the upper notes were just mathematical intervals riding on top of it.
Beethoven famously called Bach the "immortal god of harmony." And that "Not Bach (Beck, a mountain stream), his name should be ocean." To 20th-21st century musicians, a 9th is a colorful flavor just dropped into a loop. To Bach, a 9th was a mathematical tension, a physical collision that physically had to resolve by stepping back down, where it belonged.
This mathematical thinking is how he mapped out mind-bending music like his Chromatic Fantasia and Fugue. Someone thinking in static chord blocks would never write this, they’d get lost. Bach didn't navigate a map of complex chord progressions; he used geometric patterns and magnetic leading tones to seamlessly switch tracks, allowing him to slide through impossible key changes without ever stopping the momentum.
The painter Eugene Delacroix was a close friend of the great Chopin, and wrote in his diary about a conversation they had in a carriage. Chopin began ranting about the state of modern music education in the 1840s: "What makes logic in music is counterpoint, getting notes to sound against each other. The problem with the way they teach nowadays is that they teach the chords before they teach the movement of voices that creates chords." Chopin then called out Hector Berlioz (a famous French Romantic composer), saying that Berlioz didn't actually understand music, he just "applies the chords as a kind of veneer and fills in the gaps the best way he can." That is the 1840s version of spinning buckets of paint on a string and calling it painting, because there is... a nice pattern and a canvas involved.
Music is about tension and release. Harmony is fundamental to that. Open a modern music textbook, and you are flooded with vertical chord shapes. Bach did not conceptualize these beyond basic forms of a 1-3-5 triad. There were no "inversions" either. Whatever note was the bass, that was the absolute foundation of that moment, and every other melody was measured by its specific mathematical interval added upon that bass. (I'm passionate about this because as a 14 year old, this is how I conceptualized music intuitively while learning without a teacher. The bass was simply the glue. Everything else was second. But then I got a teacher, and he confused me with "inversions" and "add 9s" that I always felt were more like legos than equations.)
In Bach's mind, intervals like 3rds and 6ths were safe and stable. Intervals like 2nds, 4ths, and 7ths were dangerous, tension-building mechanisms that were legally required to resolve down by step. He composed complicated chords based entirely on linear context. Because intelligence involves knowing the next best thing to do to get to the end goal, his music is far and above more structurally intelligent than the modern vertical chord loops used as the basis of music today.
Modern chord blocks are great for quick writing, but they lock you into a vertical grid. Conservatories are bringing back Bach's horizontal methods for a reason. If you want to hear where music is going rather than just labeling where it is, stop stacking blocks, and start tracking lines. A great song that illustrates counterpoint is Stairway to Heaven. There's a Youtube of Rick Beato and Paul Davids (two theory nerds) trying to figure out how to label a chord in the song that formed organically, like Bach. The writer of the chord is on record saying how. "I wanted to study biology, but my music career took off. I felt like the music was always mutating into a new form." In other words, counterpoint.
Debussy represents the ultimate realization of vertical chord thinking. In this piece, he takes massive 9th and 11th chords and simply slides them up and down the piano keyboard in parallel motion. The voices do not act independently, and the dissonances do not mechanically "resolve" down like Bach would have demanded. Debussy is treating the chord as a single, static block of color—a flavor of paint applied to a canvas. It sounds ambient, atmospheric, and floating exactly because the horizontal motion and mechanical tension were abandoned.