The status of other animals and our relationship with them is a thread that runs through much of Nussbaum's writing. It can be traced back to her engagement with Aristotle, a natural historian who wrote extensively about biology and who conceived of human beings as animals, with our own distinctive nature and function in nature. Nussbaum's ten capabilities that should be made available for all humans included, as we saw in Chapter 4, the opportunity to relate to other species (Nussbaum 2000). She explores animal emotions in her book Upheavals of Thought , published in 2001. Also in 2001, Nussbaum reviewed a book by the legal theorist Steven Wise, Rattling the Cage. Nussbaum supported Wise's programme of greater legal rights for non-human animals – she wrote that the book made ‘an important contribution to progress on one of the most urgent moral issues of our time’ – but thought his case needed a more secure philosophical foundation: ‘Rattling the Cage, while provocative, is more of a work of activism than of scholarship. Its powerful rhetoric and compelling social message are marred by historical and theoretical shortcomings’ (Nussbaum 2001b: 1513). In 2004, she and Cass Sunstein ( a Harvard professor of law and behavioural economist) co-edited and contributed to the book Animal Rights: Current Debates and New Directions (which included an essay by Wise). Nussbaum and Sunstein were in a relationship for ten years and the book is another example of Nussbaum's fruitful intellectual collaborations. It reflected a growing international concern with our treatment of animals: as Sunstein states in his introduction, ‘since the early 1990s, the animal rights question has moved from the periphery and toward the center of political and legal debate’ (Sunstein in Sunstein and Nussbaum 2004: 4). This focus on a pressing contemporary issue is typical of Nussbaum, who has always sought to apply her considerable knowledge of classical and early modern philosophy to problems in the here and now. The book is noteworthy in Nussbaum's oeuvre for two reasons. First, she takes on the role of curator, with Sunstein, of a range of contemporary positions on animal rights, by some of the most influential theorists in the field. Second, in her own contribution she extends the capabilities approach to the question of animal rights. She goes on to develop this approach further in Frontiers of Justice (2006).