By claiming that it is morally acceptable to kill and eat animals, we are assuming that animals do not fulfil some morally relevant criterion that qualifies them as having a right to life or a right to bodily integrity. Since humans have such rights, they must have some capacity that qualifies them as morally significant. Sentience alone is not sufficient for animals to gain moral status, since we know that animals can feel pain and pleasure, but we still feel entitled to keep them as pets or to kill them for our purposes. It seems that most people think that the relevant difference between humans and animals lies in the higher mental states of the former. According to the philosopher Harry G. Frankfurt, what distinguishes humans from animals is their ability to be sufficiently rational, in the sense that humans are able to reflect on their thoughts, desires and emotions. For example, if we have a desire to eat something, we can ask ourselves whether we want to have that desire or whether that desire is more harmful to us. We can identify with desires or have desires that we consider to be external to our personality (e.g. a smoker who feels the desire to smoke a cigarette but rejects the existence of this desire and does not identify with it). Frankfurt calls beings that meet this criterion "persons". Animals lack this kind of rationality, they are called non-persons and therefore - so the common understanding - they are inferior to humans and have no right to live or to be treated as property.¹

The argument from species overlap: Not all humans are persons

Those who believe that only living beings who possess the capacity of this narrow sense of rationality – i.e. living beings who are "persons" – must now, in order to remain consistent, deny rights to certain humans. After all, many humans are not rational in the sense defined earlier; they are human beings, but not "persons" in Frankfurt's sense. The severely mentally disabled, those with advanced dementia, those in a coma, and babies and young children do not meet the morally relevant criterion for rights. Despite their lack of rationality in the morally relevant sense, these persons are nevertheless accorded moral rights. On the other hand, animals, many of whom have higher mental capacities than these people, are denied moral status. This discrepancy potentially exposes the defenders of this argument to the charge of arbitrariness. We are left with two choices: either we grant animals the same rights as marginalised humans, or we deny rights to human "non-persons". The latter option would imply that we have the authority to strip less rational humans of their equal rights. Since this consequence is morally undesirable, the only viable way forward is to recognise the rights of animals, thereby adopting an alternative criterion for determining moral status as sufficient.

The philosopher Julia Tanner summarized the argument from species overlaps, as Peter Singer originally put it, in the following manner:

P1	If rationality is decisive for the granting of moral rights, then neither animals nor certain humans (human "non-persons") fulfill the criterion.
P2	Human non-persons have moral status.
P3	If human non-persons are to possess moral status – but not animals – rationality cannot be the decisive criterion for the granting of moral rights.
P4	No other criterion can be found that gives moral status to human non-persons – but denies it to animals.
K	If human non-persons have moral status, the same must apply to animals.

What exactly is Tanner's argument demonstrating?

Reductio ad absurdum / the appeal to extremes: The reductio ad absurdum is a method of proof in logic where a statement x is followed by a false or absurd conclusion y and where the premise x is rejected since the unacceptable conclusion derives from it. In the case of Singer's argument, the unacceptable conclusion is the assertion that less rational individuals have no rights. Since this conclusion is clearly absurd, it can be inferred that the premise, which suggests that a highly developed form of rationality is the sole determinant of moral status, is false. This type of reduction can be applied to any criterion typically considered as exclusive to humans, such as self-awareness, language abilities, or the capacity for reflection, as there will always be humans who lack these abilities. To avoid the absurdity of such conclusions, the only reasonable course of action is to establish the decisive criterion for granting moral rights based on a criterion that is shared by all humans and, to a large extent, by most animals - the overlap of capacities across species.

Objections to the argument from species overlaps

The question that now arises is whether it is somehow possible to grant rights to human non-persons without also granting rights to animals. In the following section, I will show that attempts to include all humans in the realm of morally relevant living beings – but not animals – fail.

1.) Interpersonal identification and potentiality for personhood as morally relevant criteria

One way of trying to morally account for less rational humans, but not animals, is to claim that we can only identify with humans in a relevant way. The kind of identification meant is not rooted in emotions such as compassion, but in a hypothetical reference to the self.³

I.) Identification-Thesis: We can identify with other people in a way that we cannot with animals.

For example, we can identify with a person who has become a non-person as a result of a stroke of fate, because we too can end up in the same state as the victim through unfortunate circumstances. We can also identify with a very young child because we too were once a child. But we cannot identify with animals to the same degree. I can feel compassion for a cat that has been abused, and I can imagine that the cat feels pain - but there is no way that I could ever have been that cat, nor will I ever be able to understand what it is like to feel like a cat. A hypothetical reference to the self is only possible among members of the human species.⁴

The argument of identification by the hypothetical reference to the self is invalid for several reasons:

Objection One: The approach is unlikely to apply to all humans. Is it possible to identify with someone who has been severely mentally disabled by a congenital genetic defect? Such a person - unlike a child - has no potential to ever be a person, and unlike a person who has lost their personhood, they cannot lose their personhood because they never had the potential to be a person. A hypothetical reference to the self could only succeed if I could plausibly imagine that I had other parents who carried this genetic defect and that I could potentially have suffered from this hereditary disease. But this seems to miss the sense of identification discussed above. It is no longer clear why the mere suggestion that I could have had completely different parents is sufficient to confer moral status on human "non-persons". If I could have been a completely different person, why couldn't I potentially belong to a different species? Couldn't a hypothetical reference to the self work with other species, such as apes, if it has to work with humans, who lack the potentiality for personhood? Identification with human non-persons by hypothetical reference to the self fails with humans who have no potentiality for personhood.

Objection Two: It is not clear why the possibility of identification with other humans is sufficient to explain why animals do not acquire moral status. At best, the approach can explain why it is easier for us to identify with children, dementia sufferers or people who have suffered a stroke than with dogs, cows or pigs - but it does not explain why this should give rise to rights or moral status.

II.) Potentiality-Thesis: (Currently) less rational humans are potential persons or they once were persons.

The argument posits that individuals with cognitive abilities equal to or less than certain animals can still be considered morally significant because of their potential to develop into persons or because they have previously possessed personhood. However, there are several problematic aspects of this claim that warrant consideration.

• The approach to potentiality is not able to draw a clear line as to when potentiality is morally relevant and when it is not: The challenge is to determine when potentiality becomes the basis for moral obligations, which raises several dilemmas. Consider, for example, the scenario of a newly fertilised egg, where it becomes uncertain whether the use of emergency contraception is morally permissible, since it may be seen as hindering the potential for personhood. Similarly, the withdrawal of life support from long-term comatose patients raises questions about our responsibilities towards individuals who were once rational beings. To deal with this issue, it is crucial to distinguish between strict and gradual potentiality. The strict potentiality perspective treats the death of a fertilised egg cell as equivalent to the death of a child, since both have the potential for personhood. However, this view seems absurd because it contradicts the common belief that a fertilised egg has no moral status because it lacks relevant characteristics. On the other hand, the gradual potentiality approach has its own challenges. It could lead to the exploitation of marginalised individuals, which is contrary to the intentions of those who defend speciesism. If moral worth is determined progressively on the basis of proximity to personhood, the result is the absurd consequence that a child who is disabled from birth has less moral worth than one who becomes disabled later in life, because of his greater distance from the ideal of personhood. Moreover, the gradual potentiality approach implies that certain less rational individuals can be "sacrificed" for the benefit of rational individuals, for example by being subjected to painful experiments. These dilemmas demonstrate the complexity and potential pitfalls of relying solely on potentiality as a criterion for moral status.⁵
• Potentiality is given more emphasis than reality: Just because we are all potentially dead, it does not follow that we should be treated as potentially dead, and just because we were all once children, it does not follow that we should be treated as such. What matters is not so much what we are potentially, or what we were, but rather what our present status is.
• The relationship between potentiality and normative statements is unclear: As shown above, it is not clear why potentiality is suitable for determining who gets moral status and who does not. The relationship between potentiality and normative statements needs to be specified, otherwise it is not clear why my potential to become a very good guitar player does not also imply that I have an obligation to learn to play the guitar and can be criticised for not developing my guitar skills, even if I have no interest in doing so.
• All humans who are not in the present state of personhood are denied inherent value: Children, dementia patients, coma patients, etc. are only endowed with moral status because they have instantiated or will instantiate personhood at a given time. However, in their present state, e.g. in the state of "being a child", they have no inherent value (just as people in the state of "having dementia" have no inherent value), because on this account someone only has inherent value if they are currently a person.

2.) The Approach to Fairness: less rational humans are not persons because they have been treated unfairly by fate. Fairness requires us to compensate for this injustice by attributing moral status to them.

The approach to fairness is rooted in the observation that human non-persons are not responsible for the fact that they are not persons. Since we, as persons and moral agents, recognise that most marginalised people have been harmed in some way, we are obliged to provide them with a minimum of fairness to compensate for the harm. The weakness of the fairness approach is best demonstrated by looking at the underlying principle of misfortune.

Misfortune: To be in a state of misfortune means to be in a poor state due to unpredictable or unavoidable events.

If the occurrence of misfortune implies the right to fairness, it is not clear why animals are excluded from fair consideration. For it is just as trivial that a child with severe genetic retardation is not to blame for their condition it is equally trivial that a lion has not chosen to be a lion, let alone a calf has chosen that humans eat their flesh. Neither the less rational human nor the calf are responsible for their unfortunate states. To resolve this issue, it must be pointed out that human non-persons are bearers of moral rights only because they belong to a species characterized by personhood. But then again, it is unclear why belonging to a species carries moral weight, and the argument is circular, since what is actually to be proved (that human non-persons are morally significant) is already accepted or presupposed.

Another way around these problems is to argue that the reference to misfortune is misunderstood. What is meant by misfortune is neither unpredictable nor inevitable causes of suffering, but rather the notion of "not being a normal member of one's own species". Misfortune, then, is the inability of marginalised people to make use of the opportunities available to other members of their species. Fairness, therefore, only applies where someone has suffered a loss of natural potential due to external, uncontrollable factors. Since animals have not reached their status through unfortunate circumstances, but are normal members of their species, no misfortune has befallen them and no concept of fairness can be applied to them.

However, this approach still does not explain why the line should be drawn by human non-persons. It is reasonable to assume that animals can also experience misfortune in the sense that they have been denied their natural potential and cannot behave like other members of their species. Imagine a dog that has been kept on a short chain by their owner all their life. Because of their lonely, isolated existence, they have had no opportunity to socialise, let alone gain any positive experience of humans or other animals. Why should it not be reasonable to say that the dog has suffered misfortune because they have been deprived of the chance of a species-appropriate life and normal development by external circumstances beyond their control?

The principle of fairness is applied because human non-persons have been treated unfortunate by external forces or an evil has befallen them. But for fairness to compensate for misfortune, we must already assume that those affected by the misfortune have moral status. For if they did not have this status, we would not regard what happens to them as unfair. But again it is not clear why this applies only to less rational humans and not also to animals.⁵

Last but not least, it can be argued that this argument is ableist because it devalues lower or different cognitive abilities than the human standard and calls their presence a misfortune. This presupposes that a less rational person by default leads a worse life, or one that is simply less valuable, than a person with cognitive abilities that meet the standard. Although many philosophers automatically include less rational people in the list of beings worthy of moral consideration, in reality they are often discriminated against or devalued because of their alleged impairments. There is thus an overlap between speciesism and ableism.

Conclusion

Irrespective of what typically human characteristics speciesists want to tie the granting of moral rights to, there will always be some humans who do not fulfill the relevant requirements. Consequently, these people would have to be deprived of their moral status. But since this is absurd and counterintuitive, we have to choose a criterion that is shared by all humans - and therefore by most animals. Attempts to include even less rational humans in the sphere of morally significant beings, but not animals, fail. The arguments that pursue this goal are either circular or have problematic consequences.