The speaker starts out sounding like a philosopher, arguing that body and mind are linked and that both follow the easiest path, caught in a chain of cause and effect where chance and fate somehow overlap. The image of autumn leaves is the turning point: we think we steer ourselves, but we may drift the way leaves do. Living with that idea is hard, so we lie to ourselves, and nobody else can do that work for us.
Then the poem turns on its own argument. "This is not wisdom," the speaker says, calling these thoughts a kind of stitching over a guilty conscience. The tone shifts from abstract reasoning to confession. By the end, the speaker admits to knowing nothing about Truth, only what happens inside them. The mood is quiet, humble and a little sorrowful, but also calm. The speaker accepts having a small role and seems at peace with saying so.