The view he chose, and the view he didn’t
The subject could not be simpler. Matisse stood (or sat) inside his room and painted what was in front of him: a tall French window, its two leaves swung open, framing the harbor below and the sea beyond. People had been painting the view-out-the-window for centuries. What makes this one a turning point is not the motif. It is what he did to the color.
Up to this summer, the rule of European painting, even the rebel painting, was that color described the world. A sky was some version of blue because skies are blue; a shadow was a darker, cooler version of the thing it fell on. Matisse threw that rule out. He chose his colors for what they did to each other and to you, not for what the harbor actually looked like. The water is laid in with strokes that are nothing like real water; the walls of his own room are not the color of any plaster. This is the single decision that defines Fauvism: the color here is expressive, meaning it is picked to carry feeling rather than to copy what the eye reports.
He made one more decision, just as radical and easier to miss: instead of painting the whole canvas in one consistent touch, he handled each zone of the picture in a different kind of brushstroke, and let the picture hold the mismatch together by sheer color. That is the technical argument of the whole thing, made on a canvas under two feet tall: drop the job of describing the world, give it to color instead, and let color do the describing on its own terms. What that actually looks like, zone by zone, is something you have to be standing in front of the canvas to see, so that is where we go next.