Ad Reinhardt made this painting sometime in the last years of his life, using oil on canvas. Up close, the whole surface of the painting is covered in these tiny, short brushstrokes, each one a slightly different shade of green, like he was trying to find the exact right color. It's almost like he built the painting up slowly, layer by layer, patiently searching for something just beyond what he could see at first. That square in the lower left corner has the darkest green. And if you stand back a bit, you start to see that Reinhardt divided the canvas into a grid, with the greens getting lighter as you move to the right and upwards. It’s like he's testing the limits of perception, asking us to really look, to slow down and see the subtle differences that make each color unique. It reminds me of Agnes Martin, in that both artists use repetition and subtle variation to create works that are both meditative and challenging. It's not easy to make something so simple, and that's what makes it so powerful.