Mind is considered in terms of contents or processes or both. The term usually includes both conscious and unconscious events. In the case of the term animal mind, there is intense scientific and philosophical disagreement as to whether animal minds are unconscious or can include conscious events as well. In particular, even among scientists who may accede to the possibility of animal consciousness, there is great reluctance to consider the issue as amenable to scientific study. Donald R. Griffin is a particularly notable exception, who has made the issue a focus of his scientific attention.
Overview of Philosophical and Scientific History
Concerns that still strongly engage philosophers and psychologists to this day were raised by the opposing ideas of John Locke (1632–1704) and Rene Descartes (1596–1650). In Locke's accounting, the elements of mind are ideas. Ideas are written by experience onto the blank slate of the mind, the tabula rasa first proposed by Aristotle. Descartes claimed that ideas are innate.
Locke considered that human's ideas are created through sensations; furthermore human minds can reflect upon their ideas. According to Locke, an automatic process of association is an essential mechanism in the linkage between ideas. Descartes, too, proposed automatic, mechanistic connections to explain the mind and behavior of animals and much of humans'. Descartes emphasized automatic reflexes, which are connections between stimulating sensations of the external world and the organism's response to those sensations. For humans, Descartes proposed a mediating influence that could be exerted on reflexes by the soul operating through the brain. These views of Locke and Descartes strongly determined the field of experimental psychology; the reflex and the process of association formed the basis of the phenomenon of classical conditioning.
Both Locke's and Descartes's ideas impacted directly on the study of animal mind. Descartes had claimed that man has a soul, while animals do not; they are mere automata. Humans too have automatic processes, but humans are conscious, feel pain, and experience pleasure, while animals do not. Locke considered animals to have memory and to be capable of simple cognitive processes, including simple reasoning. They lack, however, the capacity to manipulate their ideas, to reflect upon them, as humans can. Essentially, "Brutes abstract not" (Locke 1690, website, p. 31).
With the advent of Charles Darwin's theory of evolution, the proposed continuity between humans and animals promoted a search for animal abilities that were precursors of human abilities. Darwin's The Expression of Emotions in Man and Animals (1872) not only proposed that animals experience emotions, but that, indeed, the expression of human emotion is in many ways similar to and derivative from that of certain other species, particularly nonhuman primates. At about that time, George Romanes (1882) compiled numerous examples of animal intelligence; the range of presumed capacities startled the public and scientists were criticized. The criticism, especially in later decades, decried the anecdotal nature of many observations and stressed the need for experimental verification. These issues continue to trouble the adequate documentation of observed instances of intelligent behavior, which would most plausibly be revealed in single, unique instances as an organism attempts to deal with a novel situation.
In the 1920s, Ivan Pavlov's study of digestion in dogs led him to discover that the dogs learned to anticipate the arrival of food via signals in the environment, such as his entry into the room. Evidence was the dogs salivating well before food was in their mouths. Pavlov's many subsequent detailed studies revealed underlying laws of classical conditioning.
The behavioristic approach was further espoused by James Watson and then by B. F. Skinner. They argued that private mental states cannot be the subject matter of science, only public events can be. Concentration was on learned behavior, reducible to stimulus-response units, which were subject to psychological laws. The laws of behavior were derivative of Locke's postulated process of association and, with the Pavlovian laws of classical conditioning, dominate experimental psychology even into the twenty-first century.
In a more cognitive approach, Edward Tolman's studies (1948) of rats learning their way through complicated mazes led him to propose that rats create a tentative, cognitive map indicating routes and environmental relationships, which determine the rats' responses. He struggled with the issue of behavioral indices of mental states. Of particular interest to him was specifying descriptive properties of a behavior to indicate that it is purposive. Tolman's views met with skepticism and interest in them faded until the concept of animals' cognitive maps was revived in an important book by John O'Keefe and Lynn Nadel (1978).
Griffin's influential book, The Question of Animal Awareness (1976), and his several subsequent books, reawakened both interest and controversy about animal awareness and thinking. His emphasis that animal awareness is an issue amenable to scientific study spurred investigations into animal cognitive capacities, both in the lab and the field.
Yet just what cognitive processes animal minds possess is controversial. Most contemporary experimental psychologists prefer to examine such processes without relevance to issues of consciousness. In an effort to create highly replicable experimental paradigms in controlled laboratory settings, the scientists can justifiably be strongly criticized for setting for their subjects very simplistic tasks, many of little or no relevance to the organism in its natural life, situating them in impoverished environments for rearing and testing (e.g. T-mazes or Skinner boxes/operant chambers) and for ignoring the contextual effects that are always part of the experimental conditions (e.g., as Pavlov had noted, the dogs in his study began salivating before his original digestion experiment had officially begun). Furthermore the subject of choice is most often the white rat, a genetically inbred docile animal, which may well have lost some of the cognitive and other traits essential to survival in the complex, treacherous world of the wild rat.
In addition to psychologists, ethologists, in particular cognitive ethologists, also study animal minds. Cognitive ethology is a field established by Griffin, being the study of animals' mental experiences, particularly in the course of their daily lives in their natural environment. Data are gathered either from observations or experiments in the field, initial observations often forming the basis for creating the experimental investigations. Likewise some laboratory work has become more naturalistic, employing larger spaces and other means to simulate the organism's niche. Philosophers of science and of mind have shown interest in the field of cognitive ethology, and some, such as Daniel Dennett and Colin Allen, have collaborated in varying degrees with ethologists. Other very relevant contributions have been made, such as Ruth Millikan's (1984, 1995) analysis of natural functions and both Jonathan Bennett's (1989)and John Searle's (2000) considerations of intentionality, belief-desire systems, and consciousness.
Capacities of Animal Mind
The aspects of animal mind include cognitive, emotional, moral, and communicative capacities and consciousness.
By defining cognition very broadly, one can include the simplest processes, for example, habituation, found in fairly simple creatures such as the sea slug, Aplysia, to processes of abstraction, inference, and deception, credited to some primates and selected other species. (Habituation is a process whereby an organism decreases responding to a repeated stimulus.) In most psychological analyses, investigators assume that processes found at the lowest evolutionary levels are similarly to be found in any and all higher organisms (insofar as a hierarchical notion of evolution is appropriate). This is the model of experimental psychologists who conduct laboratory studies of white rats and pigeons.
However neurophysiological studies of simple organisms such as Aplysia and the mollusk Hermissenda do note different biochemical and neural mechanisms that may underlie similar psychological processes (e.g., classical conditioning at the cellular level). Ethologists, too, are quick to note species specific and niche specific behavioral traits, which often depend upon specialized sensory receptors. Without the capacity to detect certain information, there is no opportunity to develop advanced cognitive capacities dependent upon such information. Thus bats can echo-locate and, thereby, in the dark navigate through obstacles and catch minute insects; dogs can follow faint odor trails of individuals; humans can do neither.
Psychologists would argue that the same basic psychological laws can be applied to different sensory systems, but there is mounting evidence against this interpretation. Rather than the laws of classical conditioning applying equally to all stimuli, evidence shows, for example, a bias for associating stimuli that are involved with ingestive systems. Thus, in laboratory experiments, rats tend to associate taste with apparent nausea (induced by X-rays) while visual and auditory stimuli are readily associated with painful exteroceptive stimuli. The latter biases are usually interpreted as associations most relevant in predatory-prey interactions and in other dangerous environmental events, as seen in work by John Garcia and R. A. Koelling (1966). Pigeons are biased to associate visual cues with X-ray induced illness; for the pigeon, vision is most essential in detecting their appropriate foods, such as grains. Simply put, organisms have evolved to readily learn which food associated stimuli make them ill, and thus are better able to avoid such. And further they can associate the stimuli with an illness occurring several hours later, contrary to assumed need for temporal contiguity.
In brief, all animal species, including some insects that have been studied, and probably even some single-celled animals, have been shown to be capable of at least the following: habituation, classical conditioning, and operant conditioning. But since the 1960s, important constraints on those basic processes have been recognized. Classical conditioning most simply refers to the learning process whereby a previously neutral stimulus (the conditioned stimulus or CS), when paired with a noxious or positive stimulus (the unconditioned stimulus or US), comes to elicit a response preparatory for or similar to that elicited by the US.
Since the 1960s important constraints on the basic learning processes have been recognized. Lab experiments showed that necessary conditions for classical conditioning were not merely those of temporal association as indicated by Locke and Pavlov. In addition, the CS had to have predictive value; thus if the US occurred too frequently not preceded by the CS, the CS was no longer predictive and much reduced conditioning occurred, if any (Rescorla 1966, 1988). These matters become of special significance when interpreting the overall cognitive abilities of animals: Are many processes most properly interpreted as simple, automatic, stamping in of associations, or should they be considered as expectancies and predictions that organisms hold about their world?
The same issues arise in considering operant conditioning, the strengthening of responses which are followed by reinforcement, or colloquially, rewards. This is basically the question: How do organisms learn how to behave in the world? Are the laws governing response learning automatic, generally applicable processes? Can animals learn behaviors without responding at all? An example might be the ability to form a cognitive map simply from observation. An early experiment had cats towed about in carts through a maze, so they never made responses to be reinforced; nevertheless the cats later could walk correctly thorough the maze. This may not seem surprising to many readers, but to psychologists intent on establishing simple, noncognitive, stimulus-response laws; this was anathema.
Animals are capable of many advanced abilities; certainly Locke was wrong in proclaiming, "Brutes abstract not." Even lab pigeons can learn natural, humanmade or even arbitrary categories. Pigeons were trained to peck for food reward at various slides including: tree/non-trees, water/non-water, people/non-people, scenes with a particular person/scenes with other people or no people, the letter A in various fonts/other letters, fish/non-fish (a natural category but not one within a pigeon's usual experience) and a random selection of fish versus non-fish versus another random collection of the same types. The pigeons succeeded at all these discriminations as indicated by differential pecking rates and were able to generalize appropriately to novel instances. Interestingly the birds took far longer to learn the arbitrary sets. And they were capable of correctly categorizing together such examples of water as a droplet or a pond.
Precisely what the pigeons were learning is open to question and beyond the scope of this limited survey. It has not been definitively demonstrated that the birds had formed concepts of tree and non-tree; they could have pecked upon detecting leafiness or trunkness; they could have refrained from pecking at various sub-groupings rather than non-tree. Numerous other studies do not resolve the issue to the satisfaction of all, though at least some species, particularly ravens, parrots, and great apes, can form concepts to criteria acceptable by very many researchers.
The study of cognitive maps in animals has produced evidence of impressive abilities. After training, pigeons shown a photograph with objects and food in it can go correctly to that location in a lab room. Pigeons that have flown around a campus can, from an aerial photograph, learn to go to designated locations, including untrained sites (Honig 1991). In bird species that cache food for the winter, numerous experiments indicate that birds not only recall the placement of hidden seeds, but they recall better those seeds which they have hidden themselves. Experiments involving displaced landmarks indicate that rats and avian species studied use geometric information from their stored representations. Chimpanzees hide stones for later use as tools, and retrieve them using near optimal paths to do so. Succinctly put, pigeons, rats, and other species have been shown, with experimental evidence, to form concepts and cognitive maps, though the precise definitions of those terms is debated.
Animal knowledge of time presents a challenge to investigating scientists. There are many reports of animals returning at appropriate times to access regularly occurring food arrivals; the most notable may be the return of bees just before tea time each afternoon to the garden tea table of the famous bee scientist Karl von Frisch. Laboratory studies indicate that rats and pigeons can learn complicated schedules of responding for food, and can estimate time durations on the order of seconds very accurately.
But there are other aspects to the knowledge of time. As the philosopher Ludwig Wittgenstein noted when discussing Locke's ideas, "We say a dog is afraid his master will beat him, but not, he is afraid his master will beat him tomorrow." (Wittgenstein 1963, vol.1, p. 650). Relevant to this concern is research with scrub jays, a species that caches food for use at a later time; the work indicates use of elapsed time information in a fairly subtle way. According to the work of Nicola Clayton and colleagues (2003), these birds can discriminate and preferentially retrieve, depending on time elapsed since storage, either a preferred food (larvae) with a shorter time until decay or a less preferred food (peanuts), which lasts longer. Some of the ape cognition and language studies do include reports by apes of past occurrences, but those data do not appropriately tackle the issue of animal knowledge of time past, present, and future. In summary, by the current two-system hypothesis, both simple, automatic learning processes and more sophisticated cognitive skills are characteristic of both animals and humans.
motivational, emotional, and moral capacities of animals
These capacities have received far less investigation than have the cognitive. Motives and emotions have been studied in the laboratory and occasionally in the field (Robert Sapolsky), particularly in reference to possible practical applications to humans. Thus theoretical and neurophysiogical/hormonal models have been proposed with regard to stress, addiction, learned helplessness, and depression. Experimental psychologists are in a dubious position, whereby some deny the possibility of animal consciousness or its scientific study, while others use animals as models for human emotions and motivations.
A possible evolutionary basis for morality reawakened research interest, beginning in the 1990s with neuroanatomical investigations and field studies. Apparent animal altruism has long intrigued scientists, resulting in theoretical models drawn, for example, from economic theorizing. Some suggest that the basis for human morality can be found in human's capacity for empathy, for understanding another's thoughts and feelings. Neurological studies confirm that merely viewing pictures of people injuring themselves, even stubbing their toes, activates some of the same brain regions that are engaged when people stub their own toes. Relevant animal research could be undertaken with potentially important results.
Griffin suggests that animal communication may well serve as a window on animal minds, and thus provide evidence relevant to animal consciousness. Comparisons are frequently made to human linguistic communication, provoking agreement and controversy. To be discussed here are both natural and artificial communication systems.
Natural animal communication systems
Late-twentieth-century research has developed beyond the rigid stimulus-response model of classical ethology and the notion that at least some animal communication is merely a by-product of an internal state, what Griffin (1992) has termed the Groan of Pain (GOP) interpretation. Central issues now concern what is being communicated.
An important approach to communication by W. John Smith (1977) stresses an interactional, informational framework, which, however, has not received adequate attention. He notes that animals' signals by themselves do not provide sufficient information to enable recipients to choose appropriate responses. The context of the signal, including the roles of the specific interactants, their past history, and the environmental characteristics, all help determine meaning. This evaluation of information implies complex cognitive processes. In Smith's view, communication importantly allows interactants to predict the other's behavior; he avoids use of intentional terms, but his analyses are indeed amenable to such.
Beginning with mere insects, one finds surprising complexity and versatility in the genetically based dance communication system of honeybees. It has been known since the time of von Frisch, from studies begun in the 1920s, that the figure eight shaped waggle dances that honeybees perform inside their darkened hive convey information about the distance, direction, and desirability of a food source, though many academic battles were fought until that information was accepted. Later research indicated the dances could convey the same information about a potential new hive location, even including site height. The dance itself seems able to persuade other dancers to change their steps, and sometimes a recipient will begin to dance about a new location, sight unseen.
Of particular interest in the continuing controversy about the distinctions between human and animal communication, is the fact that several investigations indicate that some species' signals appear to be referential, that is, the calls specify the type of predator that has been detected. The species include vervet monkeys that appear able to indicate their three major predator types: the martial eagle, the leopard or other large carnivore, and the python. Diana and Campbells monkeys likewise have two different alarm calls, one for each of their major aerial and ground predators. Even some lemurs, primitive primate-like creatures, have calls specific to raptors, as does a mongoose species.
Sometimes level of arousal is included in the information of these various species' calls. Prairie dog calls reputedly identify predator types, even conveying information about the intruder's color and size, but the research needs further verification. Note, however, that the term alarm call is controversial, for some scientists, such as Smith, emphasize the broader use of some such calls. Peter Marler and his colleagues (1986) have also investigated reference in alarm and other calls, emphasizing the role of the audience, both that present and that to which a call is directed, in determining if a signal is given and which signal is made. It is also the case that many scientists are very reluctant to accept referential use of a signal by a nonhuman animal.
Artificial communication systems
Scientists have also undertaken studies of communication in apes and other species using modified forms of human sign language, plastic chips, computerized geometric figures (lexigrams), and spoken words. It is beyond the scope of this entry to discuss these investigations fully, but it should be noted that some of the chimpanzees can respond to and produce strings of words similarly to the behaviors of a two-and-one-half-year-old human. That is not to say that the understandings of the humans and other species are the same. Whether the units should properly be termed words and whether the behavior should be termed language use is hotly debated (Terrace 1979); linguists are the strongest dissenters.
However both apes and African Grey parrots can use the communication units to indicate the color, number, and shape of items, and can accomplish cognitive tasks such as indicating same-different. Some of the apes understand and use artificial units, while also appropriately responding to some spoken English words. Apes have been reported to use the lexigrams to express simple thoughts and emotional feelings (Ristau and Robbins 1982, Ristau 1991, Savage-Rumbaugh 1998, and others).
To study consciousness, it is first necessary to delineate possible levels or kinds of consciousness, a task for both psychologists and philosophers. Since the topic is beyond the scope of this entry, note at least that consciousness can refer most simply to perceptual consciousness or awareness of sensations and pain and in more complex states to consciousness of self through past, present and anticipated future and to metacognition, or thoughts about one's thoughts and knowing that one knows. Yet even at a primitive level, it is difficult to imagine that a sensing creature, infant or animal, does not in some way distinguish between an external world and that which belongs to itself—such as its own paw.
Griffin has suggested the following as kinds of evidence for consciousness:
(1) An argument from evolution: Given that many other aspects of human structure and function are derived from those of other animal species, why should not consciousness likewise be part of the continuum?
(2) An argument from neurology: No Consciousness producing neurological structure or process can be found in humans, but absent from nonhuman animals. On the contrary, similar electrical brain waves are correlated to apparently similarly psychological functions in both humans and animals.
(3) As Griffin notes, "Appropriate responses to novel challenges for which the animal has not been prepared by genetic programming or previous experience provide suggestive evidence of animal consciousness because such versatility is most effectively organized by conscious thinking" (Griffin and Speck 2003, p. 5).
(4) Animal communication may well serve as a window to the minds of animals, revealing their subjective experiences, including intentions.
In his books and papers, Griffin (1976, 2001, 2003) reviews many experiments that provide evidence for consciousness. A few examples are noted. Beginning in the late 1970s, experiments examined the ability of chimpanzees to recognize themselves in a mirror (Gallup 1970). Children can do this after about eighteen months of age, but up to that time, they react socially to the mirror, interacting with their reflection as though it were another child. Chimpanzees also react socially, unless they have had extensive experience with mirrors. Results are mixed for other great apes, with controversial evidence from monkey species and no positive results from chickens and a myriad of other animals. Yet some monkey species, unable to recognize themselves by the mirror test, can nevertheless use a mirror to help them in a task with their otherwise unseen hand. Whatever the final evidence and interpretations, the mirror test can imply only some sense of the self as a body and not necessarily of the self as a mind, or as a self persisting from the past into the future.
A more limited claim, that rats can discriminate their own behaviors, derives from a task in which rats learned to push one of four different levers when a buzzer sounded, depending upon their own activity at the moment, for example, face washing, rearing, walking, or immobility (Beninger et al. 1974). Again interpretations of the results vary; for example, whether a rat is associating a particular lever with kinesthetic feedback from its behavior or whether a rat is indicating, " Now I am walking."
There is evidence that monkeys sometimes know what they know. As Griffin notes, "Consciously considering the contents of memory, in contrast to automatically using stored information, is a kind of metacognition, which many are still hesitant to infer in animals" (Griffin and Speck 2003, p. 13). Yet the ability to consciously consider uncertainties faced in nature is indeed an asset for an animal in a critical situation. In experiments investigating metacognition, monkeys had a choice of pressing one lever, thereby producing a less preferred food, or another lever requiring correct stimulus selection in order to receive a more preferred reward. Correct selection was difficult if monkeys had to delay their responding after seeing the stimulus they were to match. On such trials, the monkeys most often chose the less desirable reward, rather than take the test and quite likely get no reward. The author concludes that the monkeys can report the presence or absence of memory (Hampton 2001).
Creative tool making by crows, and indeed by other species, is another ability that strongly suggests conscious deliberation. There has long been considerable evidence of tool making by chimpanzees and orangutans, but less so for other species. New Caledonian crows studied in lab aviaries spontaneously used sticks to reach food in a clear cylinder, most often selecting sticks of the proper length. In other experiments, the crows selected a hooked wire, rather than a straight one to reach food most readily gained with a hook. When only a straight wire was available, the female crow, never having seen the process of wire bending, bent the wire herself to make a hook and thereby obtain the food.
Experiments have also been conducted in the field, suggesting purposeful, strategic behavior by the organisms involved. For example, Carolyn Ristau (1991) conducted experiments with piping plovers, birds that perform broken-wing or distraction displays at an intruder's approach to their nest/young. She suggested criteria for purposive behavior and found that the birds met such criteria. The plovers used the display correctly, so as to draw a human intruder away from the nest/young, positioning themselves in the intruder's front visual field. When plovers flew to reposition themselves before displaying, they went nearer the intruder and the center of the intruder's visual field. Plovers, even mid-display, monitored the intruders. Should the intruder not follow the birds' displays, the plovers modified their behaviors, re-approaching the intruder or increasing display intensity. Other experiments indicated the plovers' awareness of the direction of an intruder's attention, by becoming more aroused when a passing intruder looked toward their nest area in the dunes in contrast to looking towards the sea. Alexandra Horowitz (2002) has further developed Ristau's criteria for intentional behavior and has applied the ideas to dogs' interactive behavior.
In research by David Premack (1978, 1992), Daniel Povinelli (2000), Michael Tomasello (1997), and Frans de Waal (2003) and their colleagues, chimpanzees have been shown to be capable of complex problem solving and social understanding, sometimes interpreted as the ability to attribute and to understand other minds. Such abilities include determining the intentions of others, detecting, understanding and engaging in deception, and distinguishing between knowledge held by another in contrast to another's visual perception. Many aspects of these capacities seem reasonable evidence for consciousness.
In summary, though unresolved in the view of some, many behavioral scientists appear to be coming to agree that animals are conscious. The matter of proof of the content of mental states of any creature, human or otherwise, remains a philosophical problem. There simply are no incontrovertible means by which external behaviors, linguistic or otherwise, provide absolute proof of specific mental states. One can be certain only of one's own consciousness; this is the extreme version of the solipsistic position.
The essential problem confronting the study of animal minds as conscious entities is that of solipsism. However, in order to survive in daily life, one cannot accept the solipsistic position. In science, one can at least recognize that to declare that animals are not conscious is not a neutral stance, but one that demands proof. As Griffin notes, the probability of awareness (pA) must be assumed to be 0.5, not 0.0. So the scientific task becomes one of accumulating evidence that shifts pA in either direction, noting that level of awareness for a particular task does not necessarily imply the organism's consciousness during another task.
Several traditional philosophical lines of inquiry are to be considered in the study of animal mind, certainly the philosophy of science and of mind including the nature of scientific evidence, solipsism, nature of experience (e.g., qualia ), intentionality and gradations of belief-desire systems, linguistic concerns, nature of a referent and of representation, nature of specific cognitive capacities, and defining levels of awareness/consciousness and at least suggestive evidence for each.
Potential Roles for Philosophers
In the past, philosophers were usually dismissive of the need for scientific data in pursuing philosophical problems. Fortunately, that attitude has changed. Philosophers cognizant of the data in their area of interest can play much needed roles in elucidating unidentified assumptions in scientists' work. They can suggest the kinds of data and experimental designs required to provide insight into mental states. Philosophical examinations of Kantian and other concepts of space and time as relevant to animal minds would likewise be helpful.
But philosophers also need to accept real-world constraints on their thinking, prime among them being temporal: Organisms act in a time-limited world and often the most dangerous situations they face require making very rapid decisions. Thus organisms often operate using default mechanisms as well as more time-consuming, deliberative, or trial-and-error methods. Organisms, both animal and human, are often overloaded with information; thus simple heuristics must often suffice. Aware of constraints such as these, as well as the need to communicate effectively to those in other fields, philosophers' contributions to the understanding of animal mind can be outstanding.
See also Animal Rights and Welfare; Aristotle; Bennett, Jonathan; Darwin, Charles Robert; Dennett, Daniel Clement; Descartes, René; Locke, John; Millikan, Ruth; Pavlov, Ivan Petrovich; Qualia; Searle, John; Skinner, B. F.; Speciesism; Wittgenstein, Ludwig Josef Johann.
Beninger, R. J., S. R. Kendall, and C. H. Vanderwolf, "The Ability of Rats to Discriminate Their Own Behavior." Canadian Journal of Psychology 28 (1974): 79–91.
Bennett, Jonathan Francis. Rationality: An Essay towards an Analysis. Indianapolis, IN: Hackett, 1989.
Clayton Nicola S, Kara Shirley Yu, and Anthony Dickinson. "Interacting Cache Memories: Evidence for Flexible Memory Use by Western Scrub-Jays (Aphelocoma californica )." Journal of Experimental Psychology: Animal Behavior Processes 29 (2003): 14–22.
Darwin, Charles R. The Expression of the Emotions in Man and Animals. London: Appleton, 1872.
Dennett, Daniel C. Kinds of Minds. New York: Basic Books, 1996.
Frisch, Karl von. The Dance Language and Orientation of Bees. Cambridge, MA: Harvard University Press, 1967.
Gallup, Gordon G. "Chimpanzees: Self-Recognition." Science. Vol. 167 (3914) (January 1970): 86–87.
Garcia, John, and R. A. Koelling. "Relation of Cue to Consequence in Avoidance Learning." Science 4 (1966): 123–124.
Griffin, Donald R. Animal Minds, Beyond Cognition to Consciousness. Chicago: University of Chicago Press, 2001.
Griffin, Donald R., and Gayle B. Speck. "New Evidence of Animal Consciousness." Animal Cognition 7 (1) (2003): 5–18.
Hampton, Robert Russell. "Rhesus Monkeys Know When They Remember." Proceedings of the National Academy of Sciences USA 98 (2001): 5359–5362.
Honig, Werner K. "Structure and Function in the Spatial Memory of Animals," edited by Wickliffe C. Abraham, Michael Corballis, et al. In Memory Mechanisms: A Tribute to G. V. Goddard, pp. 293–313. Hillsdale, NJ: Lawrence Erlbaum Associates, 1991.
Horowitz, Alexandra C. "The Behaviors of Theories of Mind and a Case Study of Dogs at Play." PhD diss., University of California, San Diego, 2002. Available from http://www.crl.ucsd.edu/∼ahorowit/frontmatters.pdf.
Locke, John. An Essay concerning Human Understanding (1690). Printed by Elizabeth Holt for Thomas Basset. Reprinted by Pelagus Free Books. www.pelagus.org/books/AN_ESSAY_CONCERNING_HUMAN_UNDERSTANDING,_by_John_Locke_31.html
Marler, Peter, Alan Dufty, and Roberta Pickert. "Vocal Communication in the Domestic Chicken: II. Is a Sender Sensitive to the Presence and Nature of a Receiver?" Animal Behaviour 34 (1986): 194–198.
Millikan, Ruth Garrett. Language, Thought and Other Biological Categories. Cambridge, MA: MIT Press, 1984.
Millikan, Ruth Garrett. White Queen Psychology and Other Essays for Alice. Cambridge, MA: MIT Press, 1995.
O'Keefe, John, and Lynn Nadel. Hippocampus as a Cognitive Map. Oxford: Clarendon Press, 1978.
Povinelli, Daniel J. Folk Physics for Apes: The Chimpanzees' Theory of How the World Works. New York: Oxford University Press, 2000.
Pepperberg, Irene M. "Evolution of Avian Intelligence, with an Emphasis on Grey Parrots (Psittacus erithacus )." In The Evolution of Intelligence, edited by R. S. Sternberg and J. C. Kaufman, 315–337. Mahwah, NJ: Lawrence Erlbaum, 2002.
Premack, David. "On the Origins of Domain-Specific Primitives." Cognition: Conceptual and Methodological Issues, edited by Herbert L Pick, Jr. and Paulus Willem van den Broek, 189–212. Washington, DC: American Psychological Association, 1992.
Premack, David, and G. Woodruff. "Does the Chimpanzee Have a Theory of Mind?" Behavioral and Brain Sciences 4 (1978): 515–526 and commentaries.
Rescorla, Robert A. "Pavlovian Conditioning: It's Not What You Think It Is." American Psychologist 43 (1988): 151–160.
Rescorla, Robert A. "Predictability and Number of Pairings in Pavlovian Fear Conditioning." Psychonomic Society 4 (1966): 383–384.
Ristau, Carolyn A. "Aspects of the Cognitive Ethology of an Injury-Feigning Bird, the Piping Plover." In Cognitive Ethology: The Minds of Other Animals: Essays in Honor of Donald R. Griffin, edited by Carolyn A. Ristau, 91–126. Hillsdale, NJ: Lawrence Erlbaum, 1991.
Ristau, Carolyn A., and Donald Robbins. "Language in the Great Apes: A Critical Review." In Advances in the Study of Behavior. Vol. 12, edited by J. S. Rosenblatt, et al., 142–255. New York: Academic Press, 1982.
Romanes, George J. Animal Intelligence. London: Kegan Paul, 1882.
Savage-Rumbaugh Sue, S. G. Shanker, and T. Taylor. Apes, Language, and the Human Mind. New York: Oxford University Press, 1998.
Searle, John R. "Consciousness." Annual Review of Neuroscience 23 (2000): 557–578.
Smith, W. John. The Behavior of Communicating. Cambridge, MA: Harvard University Press, 1977.
Terrace, H. S. Nim. New York: Knopf, 1979.
Tolman, Edwin C. "Cognitive Maps in Rats and Men." Psychological Review 55 (1948): 189–208.
Tomasello, Michael, and Josep Call. Primate Cognition. New York: Oxford University Press, 1997.
Waal, Frans B. M. de, and Peter L. Tyack. Animal Social Complexity: Intelligence, Culture, and Individual Societies. Cambridge, MA: Harvard University Press, 2003.
Wittgenstein, Ludwig. Philosophical Investigations. Oxford: Basil Blackwell, 1963.
Carolyn A. Ristau (2005)