Scientists are often challenged with the so-called marginal case argument.

We are asked to spell out the criteria that make our experiments justifiable in animals but not in humans with comparable abilities and therefore comparable interests. These criteria, we are told, must be evaluated for each individual separately (so-called moral individualism). The resulting argument against animal research consists in pointing out that no matter what criteria are selected, it is always possible to find some humans (e.g., the senile, the cognitively impaired or the comatose patient) who should also be candidates for invasive research. According to this line of reasoning, logical consistency demands that we conduct experiments with these human patients along or instead of using animals. If we are unwilling to do so, then we must be guilty of speciesism.

Let me bring up a few objections to this argument.

First, it seems clear (to me at least) that the intrinsic properties of an individual cannot possibly be all that matters in assessing moral status of living beings. If such properties were all that mattered, then we should feel comfortable granting a rock, a dead cat, and human remains the same moral consideration since they can all be classified as inanimate objects with no interests of their own. And yet, while nobody will object to a child playfully kicking a rock, most will not feel comfortable with him kicking a dead cat for his or her amusement or using human remains in an art project for school. The suffering such acts will inflict on others must count as well. Thus, we must reject moral individualism. Once that premise is gone, the entire marginal case scenario falls apart.

Second, even if for the sake of argument one accepts moral individualism, the resulting moral theory is impractical. Are we prepared to evaluate every single individual we encounter in life to decide on his or her moral status? Should we assess the cognitive abilities of the child now crossing the street? The dog walking with her? The squirrel that just rushed in front of our moving car? On one hand, consistency demands that we do so, but applicability demands that we come up with a more practical approach. Indeed, our ability to function in daily life is aided by organizing the world into different categories (or kinds) of living beings and making broad assessments of their interests and moral status. Our brain’s ability to quickly recognize species membership facilitates this. It enables us to determine that the squirrel running in front of our car is a living creature and to swerve to avoid running it over—unless doing so would endanger the child crossing the street. In most situations, we can assess the interests of living beings based on the normal life of the members of that species. We have no need to assess the specific interests and moral status of this particular squirrel and this particular child.

Third, the marginal case scenario is nearly always posed by using an impaired human and a non-human animal, rather than a normal human and a non-human animal with super-natural abilities. Why? Because there is a clear difference between these two situations. On one hand, should an ape appear in front of us, such as in Kafka’s “Report to the Academy”, speaking in fluent English, asking to be treated as a peer, it seems difficult to think we could refuse on any grounds, even if it represents an extraordinary case. On the other hand, when human patients are impaired from their normal state, in most cases, we have no absolute certainty the condition is permanent. A cure for Alzheimer’s or autism may possibly be developed in the future and their mental capacities restored. Moreover establishing the lack of cognitive function with confidence may be more difficult than we have anticipated, with new studies showing that patients in vegetative state may retain some cognitive function. And, as I mentioned earlier, even in cases were science tell us there is no hope for recovery on the horizon, harming these patients would cause suffering in others that must also be taken into consideration.

Finally, there is also a scientific objection: Even if one were to accept on principle the suggestion by animal philosophers and activists that if we experiment on animals we ought to be experimenting on impaired human patients, that population would not be best suited for scientific studies. Patients with pre-existing conditions have a wide range of abnormalities and individual differences that would make it extremely difficult to conduct properly controlled scientific studies. Thus, in addition to moral considerations, there are valid scientific reasons to reject the proposal of using impaired humans rather than animal subjects in most studies.