And God said, “Let there be light,” and there was light. 4 And God saw that the light was good. And God separated the light from the darkness. 5 God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.
What God wanted, came to pass. He wanted light, so light sprang to attention. God brought light down to a formless, empty, dark world. And He separated the light from the darkness. It would seem that light and dark inherently had enough in contrast to infer their exclusivity, but God wanted to place His authoritative Word on that difference: light and dark are in fact and in function, different things.