)are often considered prima facie, meaning that the legitimacy of the obligation is accepted "on its face"; but even if not questioned, such obligations may still be ranked for ethical analysis.Thus a thief may have a negative obligation not to steal, and a police officer may have a negative obligation not to tackle people—but a police officer tackling the thief easily meets the burden of proof that he acted justifiably, since his was a breach of a lesser obligation and negated the breach of a greater obligation.