of language, and animals into discontinuous species: The director of a zoo is entitled to "put down" a chimpanzee that is surplus to requirements, while any suggestion that he might "put down" a redundant keeper or ticket-seller would be greeted with howls of incredulous outrage. The chimpanzee is the property of the zoo. Humans are nowadays not supposed to be anybody's property, yet the rationale for discriminating against chimpanzees is seldom spelled out, and I doubt if there is a defensible rationale at all. Such is the breathtaking speciesism of our Christian-inspired attitudes, the abortion of a single human zygote (most of them are destined to be spontaneously aborted anyway) can arouse more moral solicitude and righteous indignation than the vivisection of any number of intelligent adult chimpanzees! ... The only reason we can be comfortable with such a double standard is that the intermediates between humans and chimps are all dead. Dawkins elaborated in a discussion with Singer at The Center for Inquiry in 2007 when asked whether he continues to eat meat: "It's a little bit like the position which many people would have held a couple of hundred years ago over slavery. Where lots of people felt morally uneasy about slavery but went along with it because the whole economy of the South depended upon slavery." Centrality of consciousness "Libertarian extension" is the idea that the intrinsic value of nature can be extended beyond sentient beings. This seeks to apply the principle of individual rights not only to all animals but also to objects without a nervous system such as trees, plants, and rocks. Ryder rejects this argument, writing that "value cannot exist in the absence of consciousness or potential consciousness. Thus, rocks and rivers and houses have no interests and no rights of their own. This does not mean, of course, that they are not of value to us, and to many other [beings who experience pain], including those who need them as habitats and who would suffer without them." Comparisons to the Holocaust David Sztybel states in his paper, "Can the Treatment of Animals Be Compared to the Holocaust?" (2006), that the racism of the Nazis is comparable to the speciesism inherent in eating meat or using animal by-products, particularly those produced on factory farms. Y. Michael Barilan, an Israeli physician, states that speciesism is not the same thing as Nazi racism, because the latter