The argument from species overlap has been used extensively in the animal ethics literature, primarily by those who wish to defend that non-human animals, just like humans, should have their interests protected. The argument claims that the criteria that have been traditionally used to justify the disregard or disadvantageous consideration of nonhuman interests with respect to those of humans are arbitrary, since some humans also fail to satisfy them. These criteria are often the possession of certain cognitive or linguistic abilities, the possibility of being a moral agent and having responsibilities, or even the ability to have emotional bonds or to enter in power relations with other moral agents. The argument from species overlap points to the fact that many humans, such as infants and the cognitively disabled, do not satisfy these criteria either.
However, there has been some confusion as to what in particular the argument proves or does not prove, and what views it challenges. In this paper, Oscar Horta aims to bring some clarity to these issues.
The paper begins with an overview of the argument, and then considers different possible formulations of it. Horta argues that from these formulations, one of the next two conclusions follows: either 1) we accept the criteria used by those who defend the deprivation of consideration to nonhuman animals, but we must also accept that it is morally justified to treat those humans who also fail to satisfy the criteria in the same way that we currently treat non-human animals (this would include using them as resources, in particular for food). Or 2) we reject that criteria and give animals full moral consideration. This would mean that we ought to stop all animal exploitation, and that we have reasons to assist nonhuman animals when they need it, just as we do when humans need help.
Horta also argues that the scope of the argument is wider than has been previously believed. In his view, the argument not only questions those defenses of anthropocentrism (the view that only humans possess the characteristics to be the only, or primary, holders of moral standing) that appeal to capacities believed to be typically human, but also those that appeal to relationships between humans. This is because humans differ not only in their intrinsic features, but also in their relation with others. These differences affect the way they can fulfill the moral requirements that the defenders of anthropocentrism propose.
The reason why this hasn’t been noticed thus far, Horta argues, is due to the name that has been traditionally given to the argument (“the argument from marginal cases”), which he considers inappropriate for a number of reasons. The term ‘marginal’ could refer to either the deviation from a cluster of certain typological capacities that define a “normal” human being (such as walking on two feet, being within a certain IQ range, etc.), having genetic information that diverges from the standard one among humans or lacking the criteria mentioned earlier. But he argues that in none of these situations it would be appropriate to refer to a human as ‘marginal’, and that the use of the term leads to many misunderstandings. In contrast, the term ‘species overlap’ avoids these problems, and presents some advantages. It applies to the case of the criteria that appeals to relations as well as individual capacities, and better encompasses different species instead of merely pointing out the differences between them.
The paper concludes with a restatement of what the argument proves and does not prove. First, it refutes the claim that it is justified to consider disadvantageously those who do not satisfy the criteria with respect to those who do satisfy them. Second, although it does not prove that either humans or nonhumans must be considered or treated in a certain way, the argument does show that no criterion that is non-definitional (and therefore can be proved) can justify disregarding the interests of nonhuman animals without also disregarding the interests of some humans. This is very important, given that most people think no human being should be disregarded or disadvantageously considered.
Finally, the paper finishes by arguing that if all humans capable of suffering and well-being must be given full moral consideration, we might think that the reasons for this is the way they would be harmed by the actions of others were they not protected by virtue of this moral standing. This implies that having positive or negative experiences is what really matters when it comes to being morally considered. Since nonhuman animals also meet this criterion, this means that we ought to give them full moral consideration.