A pair of overlapping political perspectives arising toward the end of the 20th century are republicanism (or neo- or civic-republicanism) and the capability approach. The resurgent republican movement aims to provide an alternate definition of liberty from Isaiah Berlin's positive and negative forms of liberty, namely "liberty as non-domination." Unlike liberals who understand liberty as "non-interference," "non-domination" entails individuals not being subject to the arbitrary will of anyother person. To a liberal, a slave who is not interfered with may be free, yet to a republican the mere status as a slave, regardless of how that slave is treated, is objectionable. Prominent republicans include historian Quentin Skinner, jurist Cass Sunstein, and political philosopher Philip Pettit. The capability approach, pioneered by economists Mahbub ul Haq and Amartya Sen and further developed by legal scholar Martha Nussbaum, understands freedom under allied lines: the real-world ability to act. Both the capability approach and republicanism treat choice as something which must be resourced. In other words, it is not enough to be legally able to do something, but to have the real option of doing it.