He agreed that gratuitous cruelty to animals should be avoided, but contends that animal welfare should only be advanced where doing so provides a marginal benefit to society.
Posner argues that animal rights conflict with the moral relevance of humanity and that empathy for pain and suffering of animals does not supersede advancing society.
He rejected the state's argument that the laws were written to protect the health of women and not to make abortion more difficult to obtain.
Accusing the state of indirectly trying to ban abortions in the state Posner wrote, "They [Wisconsin] may do this in the name of protecting the health of women who have abortions, yet as in this case the specific measures they support may do little or nothing for health, but rather strew impediments to abortion." Posner rejects the concept of animal rights.
Posner was confirmed by the United States Senate on November 24, 1981, and received his commission on December 1, 1981.
He served as Chief Judge of that court from 1993 to 2000 but remained a part-time professor at the University of Chicago.
Posner opposes the US "War on Drugs" and called it "quixotic".
The Bluebook is the style guide which prescribes the most widely used legal citation system in the United States.
Posner is "one of the founding fathers of Bluebook abolitionism, having advocated it for almost twenty-five years, ever since his 1986 University of Chicago Law Review article The Bluebook: A Uniform System of Citation exemplifies hypertrophy in the anthropological sense.
and his affection for the thought of Friedrich Nietzsche set him apart from most American conservatives.
As a judge, with the exception of his rulings with respect to the sentencing guidelines and the recording of police actions, Posner's judicial votes have always placed him on the moderate-to-liberal wing of the Republican Party, where he has become more isolated over time.