Ramban

SEE, I HAVE SET BEFORE THEE THIS DAY LIFE AND GOOD, AND DEATH AND EVIL. He returns to exhort them yet again, to tell them that there are two courses in their hands and it is in their power to walk in whichever they desire, and there is no power below or above that will withhold them or stop them. And he calls heaven and earth to bear witness against them a second time, after he had said,I call heaven and earth to witness against you this day, that ye shall soon utterly perish, for now he said, I call heaven and earth to witness against you this day, that I have set before thee life and death which are the blessing and the curse, and that I have counselled you that you should choose life, that hou mayest live, thou and thy seed. Thus he is now like someone who has the witnesses sign at the end of all his words. Vayeilech