In the book, Varner defended a form of biocentric individualism, according to which all living entities have morally considerable interests.

Varner started a research project in 2001 that looked at animals in Hare's two-level utilitarianism.

The project's initial monograph, Personhood, Ethics, and Animal Cognition, was released by Oxford in 2012.

His third book was Defending Biodiversity: Environmental Science and Ethics, co-authored with Jonathan Newman and Stefan Linquist, and published with Cambridge University Press.

His doctoral research was supervised by Jon Morline, who continued as a supervisor even after leaving Madison to work at St. Olaf College.

[4] Graduating from Madison in 1988, Varner had a number of short-term jobs in the late 1980s; he lectured in philosophy at the University of Wisconsin–Stevens Point from 1987 to 1988, acted as a visiting assistant professor at Madison's Institute of Environmental Studies in the Summer of 1988, and took up the same role, this time in philosophy, at Washington University in St. Louis from 1988 to 1990.

offers a resolution of the debate between individualistic approaches to animal rights and holistic accounts of environmental ethics.

Varner defends an interest-based biocentric individualism according to which all living beings—including plants[11]—have morally significant interests that ground prima facie (though overridable) duties.

[13] This grounds Varner's argument for biocentrism, which Mark Rowlands summarises as follows: Rowlands argues that the problem with the book's central approach is that it assumes that all interests have a clear relation to welfare and thus moral considerability; an assumption which, he argues, is partially undermined by the introduction of biological interests.

[15] Jon Jensen, who reviewed the book for Ethics and the Environment, raised a similar worry, arguing that Varner did not sufficiently justify his claim that biological interests are inherently morally significant.

[19] Hare's philosophy of two-level utilitarianism has been a focus of Varner's since the early 2000s, and was the subject of his Personhood, Ethics, and Animal Cognition.

[2][7] In the book, Varner breaks with his previous biocentrism, instead endorsing sentientism (the idea that sentience is necessary and sufficient for moral considerability), prescriptivism, and two-level utilitarianism.

He argues that, according to contemporary science, vertebrates are conscious (i.e., sentient, able to feel pain), but few invertebrates are; cephalopods are an exception.

[28] This, however, applies only to critical-level thinking, and good intuitive-level theorising, he argues, would typically leave these decisions up to individuals.

[29] Varner then considers a range of proposals for sustainable, humane agriculture, including replacing cattle with buffalo and engineering blind chickens.

Varner also argues that Singer, despite the latter's advocacy for vegetarianism, presents a theory that supports certain forms of humane agriculture.