Explaining the Discontinuity Between Human and Nonhuman Minds
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BEHAVIORAL AND BRAIN SCIENCES (2008) 31, 109–178 Printed in the United States of America doi: 10.1017/S0140525X08003543 Darwin’s mistake: Explaining the discontinuity between human and nonhuman minds Derek C. Penn Department of Psychology, University of California–Los Angeles, Los Angeles, CA 90095; Cognitive Evolution Group, University of Louisiana, Lafayette, LA 70504 [email protected] http://reasoninglab.psych.ucla.edu/ http://www.cognitiveevolutiongroup.org/ Keith J. Holyoak Department of Psychology, University of California–Los Angeles, Los Angeles, CA 90095 [email protected] http://reasoninglab.psych.ucla.edu/ Daniel J. Povinelli Cognitive Evolution Group, University of Louisiana, Lafayette, LA 70504 [email protected] http://www.cognitiveevolutiongroup.org/ Abstract: 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. Keywords: analogy; animal cognition; causal learning; connectionism; Darwin; discontinuity; evolution; human mind; language; language of thought; physical symbol system; reasoning; same-different; theory of mind 1. Introduction many prominent comparative researchers have claimed that the traditional hallmarks of human cognition – for Human animals – and no other – build fires and example, complex tool use, grammatically structured wheels, diagnose each other’s illnesses, communicate language, causal-logical reasoning, mental state attribu- using symbols, navigate with maps, risk their lives for tion, metacognition, analogical inferences, mental time ideals, collaborate with each other, explain the world travel, culture, and so on – are not nearly as unique as in terms of hypothetical causes, punish strangers for we once thought (see, e.g., Bekoff et al. 2002; Call breaking rules, imagine impossible scenarios, and 2006; Clayton et al. 2003; de Waal & Tyack 2003; teach each other how to do all of the above. At first Matsuzawa 2001; Pepperberg 2002; Rendell & blush, it might appear obvious that human minds are Whitehead 2001; Savage-Rumbaugh et al. 1998; Smith qualitatively different from those of every other et al. 2003; Tomasello et al. 2003a). Pepperberg (2005, animal on the planet. Ever since Darwin, however, p. 469) aptly sums up the comparative consensus as the dominant tendency in comparative cognitive follows: “for over 35 years, researchers have been psychology has been to emphasize the continuity demonstrating through tests both in the field and in the lab- between human and nonhuman minds and to downplay oratory that the capacities of nonhuman animals to solve the differences as “one of degree and not of kind” complex problems form a continuum with those of (Darwin 1871). Particularly in the last quarter century, humans.” # 2008 Cambridge University Press 0140-525X/08 $40.00 109 Penn et al.: Darwin’s mistake: Explaining the discontinuity between human and nonhuman minds Of course, many scholars continue to claim that there is Tomasello and Rakoczy (2003, p. 121) argue that the something qualitatively different about at least some ability to participate in cultural activities with shared goals human faculties, particularly those associated with language and intentions is uniquely human, but claim that the cogni- and a representational theory of mind (see, e.g., Bermudez tive skills of a human child born on a desert island and 2003; Carruthers 2002; Donald 2001; Mithen 1996; somehow magically kept alive by itself until adulthood Premack 2007; Suddendorf & Corballis 2007a). Nearly “would not differ very much – perhaps a little, but not everyone agrees that there is something uniquely human very much” from the cognitive skills of other great apes about our ability to represent and reason about our own (see also Tomasello et al. 2003a; Tomasello et al. 2005). and others’ mental states (e.g., Tomasello et al. 2005). Notwithstanding the broad comparative consensus And most linguists and psycho-linguists argue that there is arrayed against us, the hypothesis we will be proposing a fundamental discontinuity between human and nonhu- in the present paper is that Darwin was mistaken: The pro- man forms of communication (e.g., Chomsky 1980; found biological continuity between human and nonhu- Jackendoff 2002; Pinker 1994). But the trend among com- man animals masks an equally profound functional parative researchers is to construe the uniquely human discontinuity between the human and nonhuman mind.1 aspect of these faculties in increasingly narrow terms. Indeed, we will argue that the functional discontinuity Hauser et al. (2002a), for example, continue to claim that between human and nonhuman minds pervades nearly grammatically structured languages are unique to the every domain of cognition – from reasoning about human species, but suggest that the only component of spatial relations to deceiving conspecifics – and runs the human language faculty that is, in fact, uniquely much deeper than even the spectacular scaffolding pro- human is the computational mechanism of recursion. The vided by language or culture alone can explain. rest of our “conceptual-intentional” system, they argue, At the same time, we know from Darwin’s more well- differs from that of nonhuman animals only in “quantity grounded principles that there are no unbridgeable gaps rather than kind” (Hauser et al. 2002a, p. 1573). Similarly, in evolution. Therefore, one of the most important challenges confronting cognitive scientists of all stripes, in our view, is to explain how the manifest functional discontinuity between extant human and nonhuman minds could have evolved in a biologically plausible DEREK C. PENN received a Masters Degree from Boston manner. University in Philosophy and Literary Semiotics in 1987. The first – and probably most important – step in He spent the next 15 years on the trading floors of various answering this question is to clearly identify the simi- Wall Street investment firms and in Silicon Valley as a software entrepreneur. In 2002 he retired from the larities and the dissimilarities between human and nonhu- business world to pursue his life-long interest in compara- man cognition from a purely functional point of view. We tive psychology. He is currently affiliated with the Cogni- therefore spend the bulk of the paper reexamining the evi- tive Evolution Group, University of Louisiana, and the dence for “human-like” cognitive abilities among nonhu- University of California, Los Angeles and is working on man animals at a functional level, before speculating as a trade book with Daniel J. Povinelli based on the hypoth- to how these processes might be implemented. We cover eses proposed in the present article. a wide variety of domains, species, and experimental pro- tocols – ranging from spatial relations and mental state KEITH J. HOLYOAK is a Distinguished Professor of Psy- reasoning in the lab to dominance relations and transitive chology at the University of California, Los Angeles. The inferences in the wild. Across all these disparate cases, author of more than 180 research articles, his books include Mental Leaps: Analogy in Creative Thought a consistent pattern emerges: Although there is a profound (co-authored with Paul Thagard, MIT Press, 1995) and similarity between human and nonhuman animals’ abil- The Cambridge Handbook of Thinking and Reasoning ities to learn about and act on the perceptual relations (co-edited with Robert Morrison, Cambridge University between events, properties, and objects in the world, Press, 2005). A past recipient of a Guggenheim Fellow- only humans appear capable of reinterpreting the ship, Holyoak is a Fellow of the American Association higher-order relation between these perceptual relations for the Advancement of Science, the Association for in a structurally systematic and inferentially productive Psychological Science, the Cognitive Science Society, fashion. In particular, only humans form general cat- and the Society for Experimental Psychology. egories based on structural rather than perceptual criteria, find analogies between perceptually disparate relations, DANIEL J. POVINELLI is a Professor of Biology at the University of Louisiana. He is the recipient of an Ameri- draw inferences based on the hierarchical or logical can Psychological Association Award for an Early Career relation between relations, cognize the abstract functional Contribution to Psychology, an National Science Foun- role played by constituents in a relation as distinct