The argument from species overlap

The argument from species overlap

As the section on arguments against speciesism shows, there are different ways disregard for nonhuman animals has been defended. One way is by claiming that we don’t have to fully respect nonhuman animals because they don’t have certain capacities. These capacities are typically intellectual, or intellectually related, such as the use of a language and the capacity to have responsibilities.1

Another way to justify disregard for nonhuman animals is to claim that humans should respect other humans but not animals because humans have a special relationship with each other, and they have no special relationships with other animals, or have relationships with them that are different from those shared by humans. It’s argued, for instance, that humans love other humans, or have some bonds of sympathy or solidarity with each other, but don’t have those relationships with other animals, and can therefore discriminate against them. In other cases, it’s claimed that these relationships are power relationships. That is, humans are stronger than nonhuman animals and so can disregard them as they wish, while they should respect other humans because they have a similar level of power.2

The argument from species overlap shows that these claims fail to prove that humans should be respected over other animals, and suggests that such claims must be rejected.3

The premise and conclusions drawn from these claims can be presented as follows:

(1) It is justified to deny full moral consideration to those who lack certain intellectual capacities, or who lack special relationships with others.

(2) Not all humans have certain intellectual capacities or special relationships with others.

(3) It is justified to deny full moral consideration to those humans who lack certain intellectual capacities, or who lack special relationships with others.

If we accept the two premises of the argument, that is (1) and (2), then the conclusion, which is step (3), must be accepted. Step 3 follows logically from 1 and 2. The conclusion of the argument is unavoidable.

We must also note that the second premise in this argument is irrefutable. It’s simply a fact that there are humans who don’t have certain intellectual capacities. Even if most humans have them, it remains that there are many who don’t. Likewise, there are many special relationships that many humans have, such as relationships of love or solidarity. But there are also humans who don’t have these relationships with others. This is the case with many orphans and elderly people, for instance. If the relationships in question are power relationships, there are many people who are enslaved.

All this means that there is only one way in which the conclusion from the argument can be avoided: by giving up the first premise. That is, we have to reject the view that it’s justified to give less consideration to the interests of those who don’t have certain capacities or who don’t have certain special relationships with us or others. But this, obviously, means that we can’t use this argument anymore to discriminate against nonhuman animals.

This argument shows that if one tries to defend the position that humans should be favored over other animals, one can’t do it by claiming that humans are the only ones who satisfy certain conditions, at least if the satisfaction of those conditions is something we can verify. Of course, defenders of anthropocentrism may still want to defend this view by claiming that only humans can satisfy certain conditions in a way that is impossible to verify, but that view will be rebutted by the argument against begging the question.

As a result of this, we face the following choice. We can accept that those who don’t have the aforementioned capacities and relationships should be disregarded or deserve less consideration. This means that nonhumans and many humans will not be fully respected. Or, we can reject that, and claim that in order to be fully respected one simply needs to have interests, that is, one simply needs to be sentient.

The argument also suggests that having intellectual capacities or special relationships are not acceptable conditions as a basis for deserving respect. They are not relevant requirements for it, as the argument from relevance also shows.

The argument from species overlap has often been called “the argument from marginal cases”.4 But this name is misleading and highly inaccurate (not surprisingly, it was a defender of speciesism who coined it).5 The name suggests that the cases in which humans fail to satisfy the aforementioned criteria are marginal ones. But they are not. Those humans who do not have the mentioned capacities or relationships are fully human, not half humans as the term “marginal” suggests. There are many humans in this situation, so their cases can’t be considered marginal in that respect either. It makes much more sense to point out that there is an overlap among different species regarding how they satisfy certain requirements that are often given for being respected, so those requirements can’t be said to be satisfied by all the members of a certain species (i.e., human beings) and only by them.

The argument from differences among humans

A related, but distinct argument to the one we just saw is the one we can call the argument from the differences among humans. This argument asserts that if intellectual capacities are relevant to the giving of respect, then we must accept that the amount of respect individuals receive should depend on their intellectual capacities.

Now, if this is the case, we must conclude that there are some humans who deserve more respect than others, and that no one will be respected as equals. Some humans will deserve much less respect than many nonhuman animals (the argument from species overlap also shows this).

Thus, those with outstanding abilities would be given much greater consideration than others. This means that the interests of people like Newton, Einstein, Aristotle, and Leonardo da Vinci will deserve much more attention and respect than other humans. This elitist position is not easy to accept.

But that’s not all. When we look at it on a smaller scale, if there’s a conflict of interests between two human beings and one of them has greater cognitive capacities, their interests will prevail. This argument can be presented as follows:

(1) It’s justified to treat those with greater intellectual capacities better than those with lesser intellectual capacities.

(2) Human beings have different intellectual capacities.

(3) Human beings who have greater intellectual capacities should be treated better than those who have lesser intellectual capacities.

(3’) Human beings who have lesser intellectual capacities should be treated worse than those who have greater intellectual capacities.

This is contrary to the values the majority of us have. Most people believe that all human beings should be respected equally. But as we have seen, this is something we will have to reject if we accept the idea that we can discriminate against nonhuman animals because they don’t have certain intellectual capacities.

The same argument can be used if, instead of talking about intellectual capacities, we maintain that nonhuman animals shouldn’t be respected because they lack other capacities (such as the possession of language or the capacity to respect others) because for any capacity there will always be some humans who possess it to a greater degree and others who possess it to a lesser degree, or not at all.

So, ultimately, those who consider it legitimate to treat nonhuman animals unfavorably based on their capacities cannot defend equal treatment of all humans. This seems very difficult to accept, at least for most people, and this suggests that we should change our position on this, and reject arguments that deny full moral consideration to nonhuman animals.

Further readings

Arneson, R. J. (1999) “What, if anything, renders all humans morally equal”, in Jamieson, D. (ed.) Singer and his critics, Oxford: Blackwell, pp. 103-128.

Bernstein, J. H. (1998) On moral considerability: An essay on who morally matters, Oxford: Oxford University Press.

Cushing, S. (2003) “Against ‘humanism’: Speciesism, personhood and preference”, Journal of Social Philosophy, 34, pp. 556-571.

DeGrazia, D. (1996) Taking animals seriously: Mental life and moral status, Cambridge: Cambridge University Press.

Ehnert, J. (2002) The argument from species overlap, master’s thesis, Blacksburg: Virginia Polytechnic Institute and State University [accessed on 23 August 2018].

Frey, R. G. & Paton, William (1989) “Vivisection, morals and medicine: An exchange”, in Regan, T. & Singer, P. (eds.) Animal rights and human obligations, Englewood Cliffs: Prentice Hall, pp. 223-226.

Horta, O. (2010) “What is speciesism?”, Journal of Agricultural and Environmental Ethics, 23, pp. 243-266 [accessed on 30 October 2013].

Kaufman, F. (1998) “Speciesism and the argument from misfortune”, Journal of Applied Philosophy, 15, pp. 155-163.

McMahan, J. (1996) “Cognitive disability, misfortune, and justice”, Philosophy and Public Affairs, 25, pp. 3-35.

McMahan, J. (2002) The ethics of killing: Problems at the margins of life, Oxford: Oxford University Press.

Pluhar, E. (1996) Beyond prejudice: The moral significance of human and nonhuman animals, Durham: Duke University Press.

Regan, T. (1979) “An examination and defense of one argument concerning animal rights”, Inquiry, 22, pp. 189-219.

Singer, P. (2009 [1975]), Animal liberation, New York: Harper Perenial Modern Classics.

Wilson, S. D. (2005) “The species-norm account of moral status”, Between the Species, 13 (5) [accessed on 13 February 2013].


1 Francis, L. P. & Norman, R. (1978) “Some animals are more equal than others”, Philosophy, 53, pp. 507-527. McCloskey, H. J. (1979) “Moral rights and animals”, Inquiry, 22, pp. 23-54. Leahy, M. P. T. (1991) Against liberation: Putting animals in perspective, London: Routledge. Carruthers, P. (1992) The animals issue: Moral theory in practice, Cambridge: Cambridge University Press.

2 Narveson, J. (1987) “On a case for animal rights”, The Monist, 70, pp. 31-49. Becker, L. C. (1983) “The priority of human interests”, in Miller, H. B. & Williams, W. (eds.) Ethics and animals, Clifton: Humana Press, pp. 225-242. Midgley, M. (1983) Animals and why they matter, Athens: University of Georgia Press. Callicott, J. B. (1989) In defense of the land ethic: Essays in environmental philosophy, Albany: The State University of New York Press. Petrinovich, L. (1999) Darwinian dominion: Animal welfare and human interests, Massachusetts: MIT Press.

3 This argument has been around for a long time. See for instance Bentham, J. (1996 [1907]) Introduction to the principles of moral and legislation, Oxford: Clarendon, p. 282n, and long before Porphyry (1823 [ca. 280]) Abstinence from animal food, London: Thomas Taylor [accessed on 12 November 2012]. For a detailed formulation of the argument see Horta, O. (2014) “The scope of the argument from species overlap”, Journal of Applied Philosophy, 31, pp. 142-154 [accessed on 25 October 2014].

4 Pluhar, E. (1987) “The personhood view and the argument from marginal cases”, Philosophica, 39, pp. 23-38. Dombrowski, D. A. (1997) Babies and beasts: The argument from marginal cases, Chicago: University of Illinois.

5 Narveson, J. (1977) “Animal rights”, Canadian Journal of Philosophy, 7, pp. 161-178.