Painting had always been a window
To see why Autumn Rhythm counts as a break and not just an oddity, you have to picture clearly what it broke from. Since the Renaissance, a Western painting was understood as a kind of window. You looked through the flat surface into an illusion of space, and inside that space sat a subject: a body, a saint, a landscape, a bowl of fruit. The picture had a figure and a ground (a thing, and the background it stood out against), a focal point the composition was built around, and a clear hierarchy of important areas and lesser ones. Paint described something. Even most earlier abstraction kept a composed center, an edge, a top and bottom, a sense of forms arranged in a space. The window stayed.
The window dissolves
Autumn Rhythm dissolves all of it at once, and that combination is the break. There is the allover field with no focal point, every inch weighted the same, the eye with nowhere to land. There is no figure and ground: foreground and background have fused into one continuous woven surface, with nothing in front of anything. And there is no depicted object at all: the picture is not of a thing, so there is no window and nothing on the far side of it. Take away the center, take away figure and ground, take away the subject, and what is a painting then? Pollock’s answer is the whole point of him.
The painting is the record of an event
His answer is that the painting becomes the record of an act. With nothing depicted, what the canvas actually carries is the trace of the body that made it: every poured line is the frozen track of a movement, a fling, a step around the canvas on the floor. The subject of the picture is the act of painting it. It is, in effect, the physical residue of Pollock moving through time and space over the cloth, painting as a kind of recorded performance. (This is the insight the critic Harold Rosenbergreached for, a couple of years later in 1952, when he wrote that for these painters the canvas had become “an arena in which to act” rather than a space in which to reproduce a thing. The phrase is his, and it came after; but it names what is already happening here.)
That is the hinge, and it is enormous. Once a painting can be the record of an act rather than the image of a thing, the door is open: to the great fields of pure color the next generation would pour, to the bare non-relational surfaces of Minimalism, to the whole later idea that the process of making could itself be the art. The allover field, the loss of figure and ground, and the act made into the subject, all at once, in one seventeen-foot canvas on a barn floor, is why Autumn Rhythm sits at the front of this story and not in a footnote.