Over the last quarter century, the dominant tendency in comparative cognitive psychology has been to emphasize the similarities between human and nonhuman minds and to downplay the differences as “one of degree and not of kind” (Darwin 1871). In the present target article, we argue that Darwin was mistaken: the profound biological continuity between human and nonhuman animals masks an equally profound discontinuity between human and nonhuman minds. To wit, there is a significant discontinuity in the degree to which human and nonhuman animals are able to approximate the higher-order, systematic, relational capabilities of a physical symbol system (PSS) (Newell 1980). We show that this symbolic-relational discontinuity pervades nearly every domain of cognition and runs much deeper than even the spectacular scaffolding provided by language or culture alone can explain. We propose a representational-level specification as to where human and nonhuman animals' abilities to approximate a PSS are similar and where they differ. We conclude by suggesting that recent symbolic-connectionist models of cognition shed new light on the mechanisms that underlie the gap between human and nonhuman minds.

Although Penn et al's incisive critique of comparative cognition is welcomed, their heavily computational and representational account of cognition commits them to a purely internalist view of cognitive processes. This perhaps blinds them to a distributed alternative that raises the possibility that the human cognitive revolution occurred outside the head, and not in it.

Penn et al. propose the relational reinterpretation hypothesis as an explanation of the profound discontinuities that they identify between human and nonhuman cognition. This hypothesis is not a genuine replacement for the explanations that they reject, however, because as it stands, it simply redescribes the phenomena it is trying to explain.

Although Penn et al. make a good case for the existence of deep cognitive discontinuity between humans and animals, they fail to explain how such a discontinuity could have evolved. It is proposed that until the advent of words, no species had mental representations over which higher-order relations could be computed.

Seemingly small changes in brain organization can have revolutionary consequences for function. An example is evolution's application of the primate action-planning mechanism to the management of communicative sequences. When feedback from utterances reaches the brain again through a mechanism that evolved to monitor action sequences, it makes another pass through the brain, amplifying the human power of thinking.

Penn et al. do not demonstrate Darwin made a mistake, because they largely ignore the semantics underlying the meanings of “degree” and “kind.” An analysis based on the work of Mortimer Adler shows such terminology conflates at least three different meanings of “kind,” only one of which challenges Darwin – and one which the authors almost certainly would reject.

We address the claim that nonhuman animals do not represent unobservable states, based on studies of physical cognition by rooks and social cognition by scrub-jays. In both cases, the most parsimonious explanation for the results is counter to the reinterpretation hypothesis. We suggest that imagination and prospection can be investigated in animals and included in models of cognitive architecture.

Sound comparative psychology and modern evolutionary and developmental biology emphasize powerful effects of developmental conditions on the expression of genetic endowment. Both demand that evolutionary theorists recognize these effects. Sound comparative psychology also demands experimental procedures that prevent experimenters from shaping the responses of human and nonhuman beings to conform to theoretical expectations.

We agree with Penn et al. that our human cognitive superiority derives from our exceptional relational ability. We far exceed other species in our ability to grasp analogies and to combine relations into higher-order structures (Gentner 2003). However, we argue here that possession of an elaborated symbol system – such as human language – is necessary to make our relational capacity operational.

We propose that the missing link from nonhuman to human cognition lies with our ability to form, modify, and re-form dynamic bindings between internal representations of world-states. This capacity goes beyond dynamic feature binding in perception and involves a new conception of working memory. We propose two tests for structured knowledge that might alleviate the impasse in empirical research in nonhuman animal cognition.

If our knowledge of human cognition were based solely on research with participants younger than the age of 2 years, there would be no basis for the relational reinterpretation hypothesis, and Darwin's continuity theory would be safe as houses. Because many of the shortcomings cited apply to human infants, we propose how a consideration of cognitive development would inform the relational reinterpretation hypothesis.

We dispute Penn et al.'s claim of the sharp functional discontinuity between humans and nonhumans with evidence in bottlenose dolphins (Tursiops truncatus) of higher-order generalizations: spontaneous integration of previously learned rules and concepts in response to novel stimuli. We propose that species-general explanations that are “bottom-up” in approach are more plausible than Penn et al.'s innatist approach of a genetically prespecified supermodule.

The target article provides a convincing argument that nonhuman animals cannot process role-governed rules, relational schemas, and so on, in a human-like fashion. However, actual human performance is often more similar to that of nonhuman animals than Penn et al. admit. The kind of rule-governed performance the authors take for granted may rely to a substantial degree on language on external symbol systems such as those provided by language and culture.

Relational representation abilities are a crucial cognitive difference between human and nonhuman animals. We argue that relational reasoning and representation supports the development of culture that increases in complexity. Thus, these abilities are a force that magnifies the apparent difference in cognitive abilities between humans and nonhumans.

The “rational bubble” stance espoused in the target article confounds cultural symbolic achievements with individual cognitive competences. With no explicit role for learning, the core rationale for claiming a major functional discontinuity between humans and other species rests on a hybrid formal model LISA (Learning and Inference with Schemas and Analogies) now overtaken by new models of cognitive growth and new empirical studies within an embodied systems stance.

Explaining the transition from nonhuman to human behavior is a major scientific problem. Penn et al. argue for discontinuous evolution; they review many relevant papers but miss some that disagree with their stance. Given the shifting ground on which Penn et al.'s theories are based, and the likelihood of future studies providing additional information on continuities, a more open approach to continuity is warranted.

We make three points: (1) Overlooked studies of nonhuman communication originally inspired, but no longer support, the blinkered view of mental continuity that Penn et al. critique. (2) Communicative discontinuities between animals and humans might be rooted in social-cognitive discontinuities, reflecting a common lacuna in Penn et al.'s relational reinterpretation mechanism. (3) However, relational reinterpretation need not be a qualitatively new representational process.

Data on toddler language acquisition and use support the idea of a cognitive “supermodule” that can resolve contradictory claims about human-animal similarities. Examples of imagination, aesthetic evaluation, theory of mind (ToM), and language learning reveal higher-order, relational, abstract capabilities early on. Although language itself may be a consequence of exercising this supermodule, it enables further cognitive operations on indirect experience to go far beyond animal accomplishments.

The thesis of discontinuity between humans and nonhumans requires evidence from formal reasoning tasks that rules out solutions based on associative strategies. However, insightful problem solving can be often credited through talking to humans, but not to nonhumans. We note the paradox of assuming that reasoning is orthogonal to language and enculturation while employing the criterion of using language to compare what humans and nonhumans know.

The real reason for the apparent discontinuity between human and nonhuman minds is that all closely related hominids have become extinct. Nonetheless, I agree with Penn et al. that comparative psychology should aim to establish what cognitive traits humans share with other animals and what traits they do not share, because this could make profound contributions to genetics and neuroscience. There is, however, no consensus yet, and Penn et al.'s conclusion that it all comes down to one trait is premature.