In the 1970s, the behavioral psychologist Herbert S. Terrace led a remarkable experiment to see if a chimpanzee could be taught to use language. A young ape, named "Nim Chimpsky" in a nod to the linguist whose theories Terrace challenged, was raised by a family in New York and instructed in American Sign Language. Initially, Terrace thought that Nim could create sentences but later discovered that Nim's teachers inadvertently cued his signing. Terrace concluded that Project Nim failed-not because Nim couldn't create sentences but because he couldn't even learn words. Language is a uniquely human quality, and attempting to find it in animals is wishful thinking at best. The failure of Project Nim meant we were no closer to understanding where language comes from. In this book, Terrace revisits Project Nim to offer a novel view of the origins of human language. In contrast to both Noam Chomsky and his critics, Terrace contends that words, as much as grammar, are the cornerstones of language. Retracing human evolution and developmental psychology, he shows that nonverbal interaction is the foundation of infant language acquisition, leading up to a child's first words. By placing words and conversation before grammar, we can, for the first time, account for the evolutionary basis of language. Terrace argues that this theory explains Nim's inability to acquire words and, more broadly, the differences between human and animal communication. Why Chimpanzees Can't Learn Language and Only Humans Can is a masterful statement of the nature of language and what it means to be human.

Are humans unique in having self-reflective consciousness? Or can precursors to this central form of human consciousness be found in non-human species? The Missing Link in Cognition brings together a diverse group of researchers who have been investigating this question from a variety of perspectives, including the extent to which non-human primates, and, indeed, young children, have consciousness, a sense of self, thought process, metacognitions, and representations. Some of the participants--Kitcher, Higgins, Nelson, and Tulving--argue that these types of cognitive abilities are uniquely human, whereas others--Call, Hampton, Kinsbourne, Menzel, Metcalfe, Schwartz, Smith, and Terrace--are convinced that at least the precursors to self-reflective consciousness exist in non-human primates.
Their debate focuses primarily on the underpinnings of consciousness. Some of the participants believe that consciousness depends on representational thought and on the mental manipulation of such representations. Is representational thought enough to ensure consciousness, or does one need more? If one needs more, exactly what is needed? Is reflection upon the representations, that is, metacognition, the link? Does a realization of the contingencies, that is, "knowing that," in Gilbert Ryle's terminology, ensure that a person or an animal is conscious? Is true episodic memory needed for consciousness, and if so, do any animals have it? Is it possible to have episodic memory or, indeed, any self-reflective processing, without language?
Other participants believe that consciousness is inextricably intertwined with a sense of self or self-awareness. From where does this sense of self or self-awareness arise? Some of the participants believe that it develops only through the use of language and the narrative form. If it does develop in this way, what about claims of a sense of self or self-awareness in non-human animals? Others believe that the autobiographical record implied by episodic memory is fundamental. To what extent must non-human animals have the linguistic, metacognitive, and/or representational abilities to develop a sense of self or self-awareness? These and other related concerns are crucial in this volume's lively debate over the nature of the missing cognitive link, and whether gorillas, chimps, or other species might be more like humans than many have supposed.

Human infants do not seem to be born with concepts of self or joint attention. One basic goal of Agency and Joint Attention is to unravel how these abilities originate. One approach that has received a lot of recent attention is social. Some argue that by virtue of an infant's intense eye gaze with her mother, she is able, by the age of four months, to establish a relationship with her mother that differentiates between "me" and "you." At about twelve months, the infant acquires the non-verbal ability to share attention with her mother or other caregivers. Although the concepts of self and joint attention are nonverbal and uniquely human, the question remains, how do we establish metacognitive control of these abilities? A tangential question is whether nonhuman animals develop abilities that are analogous to self and joint attention. Much of this volume is devoted to the development of metacognition of self and joint attention in experiments on the origin of consciousness, knowing oneself, social referencing, joint action, the neurological basis of joint attention, the role of joint action, mirror neurons, phenomenology, and cues for agency.