(RNS) — It may appear strange that a Catholic moral theologian (me) would interact with an atheist philosopher who has consistently rejected the sanctity of human life (Peter Singer) while demanding we humans respect the lives of nonhuman animals. But the Australian ethicist and Princeton University professor has long proved himself willing to consider and debate other viewpoints with both rigor and generosity. This has been the basis of our friendship and collaborations for more than a decade.

Singer’s claims about animals were made most famously in his 1975 book, “Animal Liberation,” a foundational text for the philosophy behind veganism. In May he published a new edition, “Animal Liberation Now,” updating his original to account for the changes in attitudes toward animals over the past half century. I talked to Singer about the new book and how Christian ethics have affected his thinking on these topics. The interview has been edited.

Why would a thinker with your views engage seriously with Christian ethics?

Philosophy is about using reason and argument to examine the foundations of our views. We can’t do that well without engaging with thinkers who start from different foundations. In ethics, in particular, if we just assume one normative theory — say, utilitarianism, or a natural law view, or Kantian ethics — is correct, we would only be describing the implications of that view, and so would not have adequate grounds for saying that our moral judgments are sound.

You ask specifically about my engagement with Christian ethics in regard to its view of the sanctity of human life. This is a topic of great importance in bioethics, with implications for abortion and euthanasia, as well as allowing people to die by withdrawing or withholding treatment. It is also relevant when we try to answer when it is wrong to kill nonhuman animals. The natural law tradition has attracted many eminent thinkers, and I can’t imagine teaching or writing on the central questions of bioethics without engaging with the best thinkers in that tradition.

You’ve shifted your views on this over the years, but is Christian ethics still the bad guy when it comes to treatment of animals?

I still think that the Christian tradition is a malign influence on our thinking about animals, but, thanks to Christians like you, as well as Andrew Linzey and David Clough, I accept that it has some positive elements as well. The problem is that the most influential Christians have written about animals as if they don’t matter at all. Take Paul’s “doth God care for oxen?” — obviously a rhetorical question to which he assumed that the answer is “no.” Or Augustine’s assertion that Jesus sent the devils into the Gadarene pigs (who then drowned themselves) in order to teach us that we have no duties to animals.

That claim influenced Aquinas, who says that we cannot sin against animals, and that being cruel to them is bad only because it might lead us to be cruel to humans. I acknowledge that Francis has rejected the view that the dominion verse in (the Bible’s Book of Genesis) means that we can treat animals as we please, but it would be good to have similarly clear statements of the need to reject the claims made by Paul, Augustine and Aquinas.

And I’m waiting for a pope to say that raising animals in factory farms is a misuse of God’s creatures, and Christians ought not to support this abuse by eating products from animals who have been treated in this way.

What reasons did you have for coming out with a new version of “Animal Liberation”?

“Animal Liberation Now” is very different from “Animal Liberation,” and that is why HarperCollins, the publisher, has given it a new title by adding the “Now.” It has to be different, because the text of the earlier book dates from 1990, and a lot has happened in the past 33 years. The first chapter describes new research on animal consciousness, and on which animals are likely to be capable of feeling pain. The two longest chapters — the second chapter, on the use of animals in research, and the third chapter, on factory farming — indicate what is happening in these areas now, instead of what was happening in the 1980s. The fourth chapter discusses ethical eating and describes some powerful new reasons for not eating meat or dairy products: slowing climate change and reducing the risk of another pandemic.

I also give more attention to the way we treat fish, a question that I neglected in the earlier editions. When we consider the vast number of fish we raise in factory farms, and the even greater number we catch and kill in painful ways, it is clear that there is no justification for the view that somehow eating fish is better than eating mammals and birds. Towards the end of Chapter 5, in which I trace the history of speciesist attitudes in Western thought, I applaud the new direction that you and the other Christian thinkers I have just mentioned are taking. In the final chapter, I outline the progress that has been made, but also what else needs to change.

What do you think is the most important thing Christians can do when it comes to respecting nonhuman animals?

Avoid factory farmed products, including farmed fish, and encourage your fellow Christians to do the same.