The first important premise of the argument is the platitude of common morality and the minimal claim of all animal activists. Roughly speaking, it says that animals are not stones and for that reason they have at least some minimal rights. I hope that only someone who believes that animals do not have any moral rights or that their suffering or killing is morally irrelevant would not accept it. However, I do not know anybody who defends such a view. Most people (at least in the USA) admit that there are morally better and worse ways of treating animals.Footnote 1 The common belief that animals should have at least some minimal moral protection can be specified in different ways due to different reasons for moral status of animals or badness of harming and killing. If someone believes that animals have at least the right not to be killed without a good reason, she should accept the first version of the argument and the following simple premise:

A1. It is morally wrong to kill sentient animals without important justification.

Different people can give different reasons to support the above claim. Many just think—especially non-philosophers—that unjustified killing is bad as a simple, intuitive, unanalyzable moral fact that does not need any further explanation or justification. This fact is sometimes stated in terms of a moral right to life: killing is bad for the same reason that it is bad to violate a right to life without important justification (e.g., self-defense). Some other people may claim that the badness of killing lies in the death that it brings. In this case, killing is wrong because it steals a life and cannot be analyzed or reduced to the loss of possible future goods or preferences satisfaction. Such a belief is sometimes associated with the thesis about the sanctity of life.

The second version of the argument is based on a different type of ethics of killing because, even if we agree that it is morally wrong to kill sentient animals without important justification, we can give other reasons for that thesis. We can claim that the badness of killing depends not on any intrinsic evil of death or sanctity of life, but on deprivation of possible future goods or preventing satisfaction of future interests and preferences (Marquis 2007; McMahan 1988; Nagel 1970; Rachels 1986). In this version if we try to assess the moral footprint of animal products we need to assess the loss involved in the death of an animal from a certain species. This loss is determined by how many future goods are taken by killing an animal. If someone believes in such an ethics of killing, she should accept the second version of the argument and the second version of premise A:

A2. It is morally wrong to deprive animals of possible future goods without important justification.

The third version of the argument is presented for persons who do not believe that killing animals is prima facie bad. It corresponds with an assumption that the badness of animal products does not lie in animal death, but in harming animals through the bad conditions they experience before their death. For that reason in order to assess the moral footprint of animal products we have to be able to measure the quantity of harm (or benefit) done to animals during their lifetime. The most obvious types of harm are pain and suffering or unsatisfied preferences that are caused by farming (Singer 1977, 2006; Thomson 1990). The same version of the argument can be used if we believe that the wrongness done to animals lies in violating their rights not to be used as property (Francione 1995; Regan 1983). In order to assess this immoral usage we need to consider, as we also must when we are focused on suffering, the amount of time spent by animals in these harming conditions, and the quantity and quality of harm done during that time. However, because an aggregation of violations of rights may engender some problems, we will focus on suffering as the more common and less controversial reason to abandon eating animal products. If somebody thinks about the badness of animal products like animal welfaristsFootnote 2 or Peter Singer, she should accept the following premise:

A3. It is morally wrong to cause suffering to animals without important justification.

Every animal activist defends at least one of the above assumptions. However, there is no need to be a moral revisionist—as many animal protectionists are—to accept any of them. Most people agree that if we can minimize animal suffering or deaths and still achieve the objective of having plentiful, inexpensive food, then we should do it (Rauch and Sharp 2005; Lusk et al. 2007). Therefore, due to the different possible answers to what “without important justification” in these contexts could mean, I assume that most of us would accept at least one of the premises (A1, A2, or A3).

The second premise (premise B) is a statement of fact about the acquisition of animal products in most developed countries. B1 (B2) are important for the first two versions of the argument.

B1. (B2.) Breeding animals for food (meat, eggs, milk) causes them to be brought into existence and causes them to be killed.

It is not practically possible to eat meat without killing an animal. Although we can imagine farming methods for non-meat products (e.g., eggs and milk) in which people are not directly involved in breeding new animals or killing those that are not economically efficient enough, almost every animal-sourced food consumed in developed countries is produced in farming conditions in which people are responsible for animal life and death. If we did not produce meat, eggs, or milk, we would not bring to life and kill pigs, chickens, and dairy cows. Every kind of animal product is involved in death of respected animals directly or indirectly.

In the third version of the argument, which is focused on the well-being of animals, we should consider one other premise:

B3. Farming affects well-being of animals positively or negatively depending on the farming method and the time spent in farms.

In order to build the third version of the argument we need to assume that different farming methods have different effect on animal well-being, either positively or negatively (for the sake of simplicity I am not going to consider animals killed in the wild). Every type of farming for every animal species causes a different quality of life. On the one hand, many farmed animals, such as pigs, chickens, dairy cows, or fish in artificial tanks, are kept in Concentrated Animal Feeding Operations (CAFOs). The standard of their life in many CAFOs is probably lower than it would be in nature. Livestock is harmed by close confinement systems (cages, crates) or lifetime confinement in indoor sheds; discomfort and injuries caused by inappropriate flooring and housing; restriction or prevention of normal exercise and most natural foraging or exploratory behavior; restriction or prevention of natural maternal behavior; lack of daylight or fresh air and poor air quality in animal sheds; social stress and injuries caused by overcrowding; health problems caused by extreme selective breeding and management for fast growth and high productivity; reduced lifetime (longevity) of breeding animals (dairy cows, breeding sows); fast-spreading infections encouraged by crowding and stress in intensive conditions; de-beaking (beak amputation without pain killer) in the poultry and eggs industry to avoid pecking in overcrowded quarters; forced and overfeeding (by inserting tubes into the throats of ducks) in the production of foie gras, etc. On the other hand, some types of farming (e.g., extensive and ecological farming of beef cattle) can probably give a better standard of life than cattle could experience in the wild.

In addition to the above premises we also should assume that:

C. Bringing an animal into the world is not morally better than not bringing an animal into the world.

It could be said that farming animals for food is good for animals even if they suffer. Meat production gives animals life and without this production these animals would not exist at all. However, such a claim and its implications are quite odd because comparing existence with non-existence produces many philosophical problems that it would be better to omit here. Moreover, even if causing an animal to exist can benefit that animal, nothing follows about the permissibility of killing it or causing it to suffer once it exists. First, causing an animal to exist benefits it only if its life is worth living, which arguably is not always the case for animals that are kept in CAFOs. Second, causing an animal to exist might entail a responsibility to care for it, not a permission to kill it. Third, once an animal exists, it may have rights that protect it, independently of how it came to exist. Fourth, it may be that once an animal exists, its interest in continuing to live outweighs any human interest that might be served by killing it. The weighing of interests seems independent of how the animal came into existence. In short, premise C is a stronger claim than I need in the argument. My argument should be persuasive even to most of those who deny premise C because of the considerations I have just noted.Footnote 3

A fourth important premise (premise D) can be roughly stated as the claim that the quantity of victims matters morally. Its validity depends on accepting premise A, because if somebody does not believe that killing or harming animals without important justification is immoral, she will not agree with premise D either.

If we admit that what makes animal products morally bad is a badness of killing animals (A1), which is the obvious precursor of eating meat and, most often, also a consequence of the production of eggs and milk, we need to assume the following premise:

D1. It is N times morally worse to kill n times more sentient animals (if everything else remains the same).

If we believe in the second reason for protecting animals (A2) and therefore focus on the second version of the argument, we need to accept:

D2. It is N times morally worse to deprive n times more possible future goods (if everything else remains the same).

The last version of premise D for the third version of the argument is focused on suffering or harm during farming:

D3. It is N times morally worse to cause n times more suffering (if everything else remains the same).

Let us imagine three cases in which you have only two options and everything else remains the same: to let 2 or 3 people die; to let 3 or 4 people die; to let 4 or 10 people die. The above premises assume that the first options are morally better and letting more rather than fewer people die is worse. If everything else remains the same, and the only thing that can be changed is the duration of feeling pain, the better option is to feel pain for as short a time as possible. The same holds true for animals. If we can test some medical research with an option that kills fewer laboratory rabbits and everything else remains the same we ought to take that option. Fewer total deaths and loss of possible future goods or pain are better than more. For every person who accepts an utilitarian ethics, the above thesis has to be perfectly obvious. In addition, most non-consequentialists would agree that if everything else remains the same, numbers morally count. On the other hand, anybody who would claim that numbers do not morally count will not accept my argument. However, only a few philosophers would defend such a view about people (Taurek 1977; Timmermann 2004) and it would be even more uncommon to claim this about animals.

The last conceptual premise (E) is important to allow cross-species moral comparisons. The formal structure of its more detailed versions (E1–E3) is:

E. The badness of killing or causing suffering to an animal from species a is m times as much as the badness of killing or causing suffering to an animal from species b.

Not every animal has the same right to life and the killing of any of them is not equally bad. Most people have different attitudes toward killing parasites, pests, livestock, wild animals under protection, or pets. In Poland people much more value the lives of cats and dogs than those of pigs, chickens, and cows. We do not kill homeless domestic animals, but put them in an animal shelter. Nevertheless, we are very merciless to rats and mosquitoes. We do not think that the death of every compared animal species is equally bad. Moreover, there are natural differences in animal bodies and psychology. As Jeff McMahan (2002) argues, psychological capacity is related to the degree of psychological (prudential) unity within a life and that psychological unity is relevant to the assessment of the harm or loss involved in death. This psychological capacity is also relevant to an animal’s capacity for well-being. Some animals are capable of much higher levels of well-being than others are capable of. Even if fish can feel pain their lost future goods are less valuable than the future goods of killed pigs, as the life of cognitively more developed animals is richer, more worth living, and worse to be shortened. Additionally, not every species has the same capability to feel pain. The nervous system of fish is not as developed as that of cows, so they could not be harmed as much by farming as cows. It can be argued that carp living in crowded tanks suffer less than pigs living alone in small steel and concrete cages. Similar treating of different animal species can produce different amounts of harm.

The premise E and the above reservations show that to make a final and detailed comparison of the moral footprint of animal products, we would need to collect a lot of empirical data about the compared animals’ psychology, physiology, and farming condition in order to assess the proper amount of harm caused by killing or farming condition of animals from different species. This is beyond the scope of this paper. However, in order to justify the thesis of the article that the ethics of consuming animal products ought to consider not just animal harm but also other factors such as animal body weight or life duration in farms, then initially we need to ignore these differences and for the sake of simplicity assume that killing or harming animals of different species is important equally. For that reason let us assume that (accordingly to the version of the argument):

E1. The death of every farmed animal is equally bad.

E2. For every farmed animal in an equal period of time of being dead the quantity and value of a loss of possible goods are equal.

E3. For every farmed animal in an equal period of time the quantity and quality of suffering are equal.

The premises E1–E3 are inconsequential. However, if we assume them for a moment we can later show that even if we know the relative physiological capacity of animals to feel pain and the real amount of harm or benefit caused by farming, this will not override the more important factors in assessing moral footprint of animal products such as animal body weight or longevity in farms. If we accept E1–E3 we will be able to see that animal capacity of well-being, amount of suffering, and harm is not as important as it is commonly believed.