People often say that humans are omnivores in order to justify eating meat as normal and vegetarianism as abnormal. The ‘Omnivore's Argument’ is one of the arguments that vegetarians and vegans encounter when meat-eaters try to defend the moral acceptability of their lifestyle choice. When responding to this argument, the position of the vegan is similar to the atheist who time and again is confronted with the same fallacious arguments in support of the existence of god(s). I am aware that it is completely in vain to attempt a refutation of these arguments, but, as a philosopher, I have devoted my life to rational arguments. Another similarity between atheists and vegans is that the burden of proof is logically on the other side: those who claim that there is a god have to adduce supporting evidence and those who claim it is moral to use nonhuman animals – the meat eaters – have to provide rational arguments in favour of including body parts on their plates. Veganism and atheism are both ethical default positions. Similarly, not killing other people is also a default position. If you kill your neighbour, you are obliged to produce a very good argument (e.g. ‘it was an accident’, or ‘it was self-defence’). If these arguments are unconvincing, you end up in jail (presupposing a morally just society). But, in the real world, the burden of proof has been shifted, because of two reasons. Firstly, the historical argument: people have always believed in god/ eating meat. Secondly, the commonality argument: many people believe in god/many people eat meat. Both arguments are logical fallacies, which becomes clear by providing one counterexample: during most of human history slavery was common and most people did not see it as a moral evil, including Plato and Jesus.

There are, I think, four sub-arguments of the Omnivore's Argument: 1) ‘It is natural’, 2) ‘It is normal’, 3) ‘The vegan lion’, and 4) ‘It is healthy’.

Let's start with the first: ‘Meat eating is natural.’ It is pointed out that humans have evolved into omnivores – this is supported by our canine teeth and the structure of our intestines. But, take a look at a close relative of Homo sapiens, the gorilla. Gorillas have huge canines and prosper on an herbivorous diet. Teeth do not always give an accurate description of food consumption habits. Chimpanzees, other close evolutionary relatives of humans, predominantly eat leaves and fruit, and only occasionally eat meat, if they can lay their hands on it. Homo sapiens is an omnivore; which means we can live and thrive on a wide range of food patterns (except the modern western obesogenic diet). A large part of the human population still does not eat meat, and certainly not daily. There is no biological necessity to eat meat. Meat eating might be natural, but the way modern meat products are processed (i.e. factory farming) is certainly not natural. Another line to defeat the ‘it's natural’-line of argument is to point out a plethora of things that are very hard to define as natural, but which most meat eaters nevertheless do. Take, for example, brushing one's teeth. That is profoundly unnatural. Most people in the history of humankind, and no animal, brushes its teeth, let alone with a brush and toothpaste. Or, if the meat eater is a biologist who comes up with examples of carnivorous crocodiles that have their teeth cleaned by helpful little birds, medicine. Modern medicine is the result of centuries of non-natural laboratory research and industrial processes – and still, meat eaters don't abstain from modern medicine because they find it unnatural. The same argument applies to such ‘unnatural’ activities as cutting our hair, using contraceptives and cooking our food (including meat).

‘It's normal to eat meat.’ Yes, indeed, it is. Most people eat or want to consume animal products. The consumption of animal products increased immensely within the past few decades, as has the practice of factory farming. However, again the example of slavery shows what humanity considers normal at one stage of societal development can drastically evolve. Normal, or socially accepted, does not automatically connote moral goodness. The task of moral philosophers should be to find moral blind spots in society, and to try to overcome them. Anthropologists, sociologists, historians and archaeologists tell us that warfare is natural for humans. Humans (men, to be precise) always went to war. Killing and raping outsiders is undeniably ‘normal’ based on historical evidence. But does that make it good? This is an example of David Hume's renowned is/ought distinction: it is not possible to draw a logical conclusion from a state of affairs (‘rape exists in all cultures in all times’) that this practice is morally good (‘rape is good’). Philosophers ponder about what is good and what arguments can support it (e.g. good is that which does not harm others and thereby contributes to the happiness of all concerned).

‘If it's wrong to kill animals, should lions and other predators stop eating other animals too?’ It will likely be impossible to convert a lion into a vegan, even in captivity, because the lion is not an omnivore, but rather a carnivore, which limits its dietary range. But what does this have to do with what humans eat? Ethics is limited to humans and other sentient creatures capable of moral deliberation, like intelligent aliens (if they exist). Simply put, moral agents who have the ability to deliberate on their actions have a moral duty to do so. We can choose how we treat animals in farms. What wild animals do or do not do is outside the range of ethics. But most importantly, what others (including lions) do does not affect how we ought to behave. If most people would own slaves, it would still be immoral for each of us to enslave others.

‘Humans need to consume meat and other animal products to be healthy.’ This argument never gets off the ground because it is the weakest. According to the American Dietetic Association, carefully planned vegan diets are healthful and nutritionally sufficient for individuals of all ages, including pregnant women, children, adolescents and athletes. There are vegan Olympians (including multiple gold medallist Carl Lewis), bodybuilders and triathletes. However, most significantly, the majority of the world's population lives on either vegetarian or vegan diets. If anything is clear, it is that we do not need to consume animal products in order to maintain our health. So, KO in the first round for the vegans. Time for some more vegetables.