The granting of parole to an eligible prisoner rests in the discretion of the U.S. Parole Commission. As prerequisites to a grant of parole, the Commission must determine that the prisoner has substantially observed the rules of the institution or institutions in which he has been confined; and upon consideration of the nature and circumstances of the offense and the history and characteristics of the prisoner, must determine that release would not depreciate the seriousness of his offense or promote disrespect for the law, and that release would not jeopardize the public welfare (i.e., that there is a reasonable probability that, if released, the prisoner would live and remain at liberty without violating the law or the conditions of his parole).