Penn et al. provocatively critique some evidence for human-like mental processes in nonhuman animals. They also claim that Darwin was wrong in asserting that the difference between the minds of humans and nonhumans is “one of degree and not of kind.” They insist that “the profound biological continuity between human and nonhuman animals masks an equally profound functional discontinuity between the human and nonhuman mind” (sect. 1, para. 3). There are many problems here. The mind is separated from biology, function is confused with underlying mechanisms, and there is a stereotypical human mind and a single type of nonhuman mind. Does the bonobo “mind” not differ from all non-bonobo minds in both degree and kind? What do “degree” and “kind” really mean anyway?

The complete sentence containing the phrase Penn et al. use to support their claim comes at the end of the summary of Chapters 2 (mental powers) and 3 (moral sense) in Darwin's The Descent of Man (1871). Darwin begins by acknowledging “that the difference between the minds of the lowest man and that of the highest animal is immense” (Darwin 1871, p. 104). He follows with a hypothetical interview with an ape, in which the latter admits his limitations in terms of being able to make tools, communicate about ideas other than about stimuli and motivations, engage in metaphysical speculation, solve mathematical problems, reflect on God, or extend moral concern to all living creatures. Then Darwin adds, “Nevertheless, the difference in mind between man and the higher animals, great as it is, is certainly one of degree and not of kind” (Darwin 1871, p. 105).

Research over the last 40 years has shown that Darwin actually underestimated the mentality of apes, for example, in tool making, numerosity, and communication. Why question Darwin now when his case is stronger than ever? In evaluating the disparaging view of reptile behavior at the dawn of the romantic dinosaur renaissance, I wrote,

Does this plaint apply to the motivations of the target article's authors? I do not know, but I fear that the consequences are indeed similar.

Until 40 years ago, Darwin's continuity claim was ignored by philosophers and humanists. Mortimer Adler was prescient in anticipating, in light of Darwin's claims, the challenge to human distinctiveness presented by the emerging evidence for unexpected levels of cognitive achievements by animals. He provided a rigorous, relevant, but unfortunately forgotten, analysis (Adler 1968).

Adler laid out four alternatives for explaining differences. First, a difference may be one of degree. Species X has more of alpha than does species Y, and an infinite number of Zs with varying degrees of alpha connect X and Y. This is the default continuity position assumed by Penn et al. Second, there are “apparent differences in kind” that are really continuities. Species X has trait non-alpha, whereas Species Y has a high level of alpha. Adler notes that the non-alpha trait is really alpha at such a low level that it appears to create a qualitative difference, but absent intermediaries could fill the gap. Third, there are “superficial differences in kind.” Suppose a threshold is reached in some beta that leads to the discontinuity in alpha resulting in species X being qualitatively different from species Y. For example, water and ice constitute real differences in kind. One is a liquid, the other a solid – a distinction in alpha/non-alpha with great functional importance. Here continuous variation in temperature (and hence molecular activity), the beta, leads to the qualitative difference in alpha. Similarly, Adler points out, one can postulate that when a certain threshold in neural or motor processing is reached, qualitative mental differences appear. This, I believe is the position of Penn et al., whereas the continuity claimants are more likely to opt for type 2. So the debate may be between those who claim that seemingly qualitative differences are just apparent versus those who claim they are real but nonetheless superficial.

The fourth alternative Adler posits, “radical difference in kind,” is due to completely new, not even emergent, properties. Adler discusses living versus nonliving. Most scientists view this as a real, but superficial, difference in which organic processes fall below the threshold to maintain life, whereas vitalists positing something special and nonmaterial in living organisms, such as the soul or vital principle, view this as a profound and radical difference in kind.

Similarly to the vitalists, Adler concludes that humans and nonhumans differ radically and not superficially. What makes humans radically different in kind are language and thought. The difference must be “psychologically explained by the operation in man and in man alone of an unobserved factor” or simply the power of conceptual thought (Adler 1968, p. 142, emphasis in the original). Adler, although admitting he may be wrong, could not conceive how animals could possibly have acquired through natural processes the psychological abilities that underlie mental differences in kind between human and nonhuman. He even anticipates Penn et al. by claiming that “perceptual abstractions are the only psychological constructs needed to explain the learned behavior of animals” (pp. 154–55, Adler's emphasis).

Adler opted for a nonmaterial factor underlying differences between human and nonhuman mentality because he feared the consequences to morality and religion if the wall of separation was not maintained. In this he has a modern compatriot in the psychiatrist Will Gaylin, cofounder and former President of the Hastings Center. The opening quotation from Gaylin shows the depths to which such thinking can descend. Creationists and intelligent design advocates will find a wedge here. Penn et al. are acting similar to those who call for “missing links” and then raise the bar whenever they are found.

Did Darwin make a mistake? I do not think so. Any mistakes lie elsewhere.