The argument follows from the more-or-less universally accepted notion that it is wrong to kill. If you told an average person that they would have to kill a lamb with their own hands before they could eat a lamb chop, they would usually respond, “No thanks, just serve me the vegetables.”

This human instinct was common among the ancients. Pythagoras, Diogenes, Socrates, Plato, and Epicurus are all believed to have been vegetarian. Two thousand years before Darwin they already knew that animals are like humans. If it was wrong to kill humans, it was wrong to kill animals unnecessarily.

Now in our post-Darwinian world we know why animals are like humans. It is because humans are animals. Deckers is perhaps the first moral philosopher to develop a conceptual framework to enable us to apply human moral responsibility beyond the human, to embrace all conscious beings. He does this through a concept called Global Health Impacts (GHIs).

Historically, moral philosophy has proceeded from the utilitarian idea of “the greatest happiness of the greatest number” (Bentham) through “do as you would be done by” (the Golden Rule) or its elaboration in the form of “act only according to a maxim by which you can at the same time will that it become a general law” (Kant). Combining these injunctions we might say that people should pursue their own happiness and the happiness of other people. We should rule out any action that constricts the pursuit of happiness by others, even if it might enhance our own.

These ideas of happiness and of the rules governing its pursuit by human beings have nothing to do with other animals, and still less to do with the environment or the ecosystem. It does not make sense to talk about a ‘happy’ ecology, but it makes eminent sense to refer to a healthy biosphere. To critique the moral responsibility of human actions that are causing the sixth mass extinction event in Earth’s history requires a concept of holistic health, which Deckers’ GHIs provide.

Deckers argues that holistic health is a prerequisite of happiness, so we should pursue health. Happiness will naturally follow. This is because GHIs refer to both physical and mental health, include both positive and negative aspects, and embrace the interests of all conscious beings, not just the human component.

For example, if a person eats meat, both positive and negative impacts accrue. They may feel good because they are eating. They may at the same time feel bad because they know they are supporting an industry that inflicts incredible amounts of pain, suffering and death on vast numbers of their fellow beings. But from the point of view of the animal whose flesh they are eating, the impacts are all negative. On balance, therefore, the negative GHIs more than outweigh the positives. If people wish to minimise negative GHIs, they need to adopt the position of qualified moral veganism.

Deckers is at pains to point out that his position would not be tenable if it could be shown that vegan diets are less healthy than omnivorous ones. But, in a detailed appendix on health findings from the medical literature, he shows that this is not the case. In fact, there is good evidence that vegetarians live longer than omnivores. Vegans who are careful to include the necessary supplements in their diets will probably live even longer than vegetarians.

There is a deep natural instinct in humanity to avoid killing, as far as possible. This is reflected in the teachings of many religions. In its most complete form it is found in Jainism. To a lesser extent it is emphasised in forms of Buddhism, Taoism and Hinduism. Even Judaism and Islam have restrictive rules around particular forms of meat-eating.

Deckers’ philosophy is not derived from such transcendental religious sources. Rather, it is founded on a respect for and understanding of the integrity of Nature, and an application of human reason (the one attribute of our species that is, arguably, unique) to a life-affirmative universal moral code adhered to by all life forms, within the limits set by their particular genetic endowments.

Whereas some interpretations of biblical texts separate our kind from others and claim a divine licence to dominate other kinds, Darwin argued that human beings are animals and that they evolved like other animals, one kind among other kinds. Deckers argues finally that an unsuppressed awareness of this close connection suggests that “it might be kind to avoid eating animal products in many situations as it really is kind to be kind to our kind.”

David Dobereiner is the author of The End of the Street: Sustainable Growth within Natural Limits.