Becoming is equal to the visible world. The sun is not itself coming to be (509b). The sun illuminates things and images but it itself is being rather than becoming. Being then can be seen as equal to the intelligible world. The sun is the realm of being.
Role of becoming and being in Divided Line: Becoming are those images that exist in real life which are compared to ideal forms which are in everyone’s understanding (510d). Being is those items that are not changing, examples being a perfect circle or a square which are both unchanging. He then says that all objects can be compared to this ideal being (VI.510c). When we think about a perfect circle or square we have an image come to mind for they are always the same. Thus there is an ideal being for all things.
Role of becoming and being in the Cave: The sun in the Cave is the being. Northing in the cave is truly unchanging in reality (VI. 515a). Things that are real one moment like shadows, people, and obscurity may not be real in the next minute thus they are becoming rather than being. The sun is stable and it provides the seasons throughout the years. The sun is the being, for it causes for everything else and it never changes