© 2000 by The Society for Integrative and Comparative Biology
Animal Consciousness: Some Philosophical, Methodological, and Evolutionary Problems1
1 Department of Biological, Anthropology and Anatomy, Box 3170, Duke University Medical Center, Durham, North Carolina 27710
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SYNOPSIS |
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No consensus exists concerning the mechanisms, distribution, or adaptive significance of consciousness. Agreement on any one of these issues would aid in resolving others. Given a reliable behavioral or neuroanatomical test for consciousness, we could map its distribution and describe its evolution. Conversely, if we knew its distribution, we could assess its adaptive value and look for similarly distributed neuroanatomies to help us get at its mechanisms.
Morgan's Canon—the rule that we should avoid attributing humanlike mental states to other animals whenever possible—impedes the use of the comparative method in unraveling this knot. If interpreted in this context as a parsimony criterion, Morgan's Canon is logically equivalent to epiphenomenalism. It is parsimonious if and only if conscious mental events play no causal role in human behavior and human consciousness has no adaptive significance. Rejecting this conclusion entails rejecting the parsimony interpretation of Morgan's Canon.
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THE PROBLEMS OF CONSCIOUSNESS |
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The phenomemon of consciousness has been a central issue in much of post-Cartesian philosophy and psychology, and the amount of attention devoted to it by philosophers and scientists has increased markedly in recent years. During the past decade, a growing number of specialists on the subject have published dozens of books on the problems of consciousness (e.g., Edelman, 1989
, 1992; Penrose, 1989, 1994; Hannay, 1990; Sommerhoff, 1990; Dennett, 1991; McGinn, 1991; Flanagan, 1992; Rosenfield, 1992; Humphrey, 1992; Searle, 1992, 1997; Herbert, 1993; Kirk, 1994; Strawson, 1994; Tye, 1995; Carruthers, 1996; Chalmers, 1996; Hobson, 1996; Lycan, 1997; Baars, 1997; Norretranders, 1998). Innumerable journal articles, meetings, and proceedings of symposia have focussed on the ontology, epistemology, mechanisms, and distribution of conscious awareness, and learned journals devoted to the topic (Journal of Consciousness Studies, Consciousness and Cognition) have come into being. Despite all this thoughtful attention, the phenomenon of consciousness is still curiously slippery and resistant to scientific study. No other biological phenomenon has remained so persistently ensnarled in fundamental philosophical and semantic tangles. As long as we avoid thinking about it, the concept of consciousness seems intuitively clear. The ebb and flow of consciousness is a familiar, everyday fact of human experience: we lose consciousness every night when we fall asleep, and regain it every morning when we wake up. Under ordinary circumstances, we are not in doubt about whether we are conscious or not. We even have little difficulty in determining when other people lose or regain consciousness. But when we try to describe just what it is that they have lost or regained, we find ourselves wandering in a maze of gray areas, circular definitions, and neurological and behavioral blind alleys.
The mechanisms of consciousness remain profoundly obscure in a way that the mechanisms of other biological phenomena are not. To be sure, it is clear that consciousness depends on the operations of the brain. Consciousness fades or vanishes when the normal patterns of waking neural activity in the brain are replaced by the patterns characteristic of epilepsy or sleep, or when the tissues of the brain are infiltrated by intoxicants or injured. Injury or disturbance of other organs engenders no such loss, unless the brain is secondarily affected by their disruption. But we have no explanation of how brain activity produces consciousness. Worse yet, we lack any real model of what such an explanation might look like. We are not thus handicapped in the case of other persistently mysterious biological phenomena. For example, we do not yet have any convincing account of how life came into being; but we can see in principle what steps would be necessary to transform assemblages of nonliving organic molecules into self-reproducing systems. In the case of consciousness, we do not even see in principle "how technicolor phenomenology can arise from grey soggy matter" (McGinn, 1989).
Some people have given up and reached various despairing conclusions. Block (1995) concludes that consciousness cannot be defined other than circularly, via rough synonyms ("awareness," etc.) or ostensively, through examples. Chalmers (1995) contends that consciousness eludes scientific definition because conscious awareness, like mass, charge, and space-time, is an elemental component of the universe not susceptible to further dissection or analysis. Skeptics like Nagel and the so-called "new mysterians" (Flanagan, 1991, p. 313) doubt that consciousness can ever be explicable in naturalistic terms, and intimate that to attempt such explanations involves a fallacy akin to the naturalistic fallacy in ethics—that it is just as logically impossible to squeeze first-person experience out of a series of third-person statements as it is to squeeze a moral imperative out of a string of facts. And both metaphysical skeptics on the one hand (Nagel, 1974; p. 437) and hard-nosed behaviorists on the other sometimes say that consciousness is not a proper subject for scientific investigation, because science is incapable of dealing with subjective, first-person phenomena.
Güzeldere (1997, p. 25) states the general problem in this way:
Consciousness just does not seem to be the kind of phenomenon that is amenable to the sort of scientific explanation that works so well with all other biological phenomena, such as digestion or reproduction. The facts that would settle the question of whether some organism—an animal or a fellow human being—is digesting do not seem to be available in the same way when it comes to the question of consciousness in others, especially in the case of organisms phylogenetically distant from ourselves ... These are the limitations of the third-person perspective: from the outside, first-hand exploration of the consciousness of others just seems to be out of the reach of ordinary scientific methods, others' experiences being neither directly observable nor noninferentially verifiable.
As far as human consciousness is concerned, the first-person problem may be something of a red herring. A scientific approach to the world is perfectly compatible with accepting the reality of subjective experience. It has to be compatible, because all experience is subjective—or, to put it another way, all observations are necessarily from the standpoint of some observer. What science demands of experiences that claim the name of evidence is not that they be "objective" in the sense of being observer-independent, but that they be replicable. Science refuses to credit unique or privileged experience. (For this reason, science is fundamentally anti-authoritarian.) But it has no difficulty dealing with subjective experience per se. Scientific experiments are not necessarily public events. They are simply experiences that any of us can have if we follow the directions; and the fact that an experience is ontologically private does not exclude it from scientific investigation.
For example, any observer with normal vision who looks fixedly at the letter "a" in Figure 1 for about 20 sec and then looks over at the letter "b" will see a circular gray afterimage surrounding the "b." Such afterimages are reliably replicable and perfectly amenable to scientific study. Their physiological basis is reasonably well understood; they result from the saturation or bleaching of retinal photopigments under intense or prolonged illumination (Brindley, 1963). Yet although afterimages have intelligible physical causes, and may even allow us to "see" details not discernible in the primary visual image (Adelson, 1982), the afterimage of the gray disk is an illusion that corresponds to no gray object "out there" in the world of noumena. Moreover, the fact that we see it is not something that we could have inferred from the facts about photopigment saturation. After all, our visual system might have evolved to eliminate afterimages by correcting for such saturation, as it eliminates and fills in the "blind spot" corresponding to the optic disk on the retina. In the present state of our knowledge, afterimages are therefore ontologically subjective phenomena.
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Although the replication of ontologically subjective phenomena is not dependent on behavioral criteria of their occurrence, identifying it as replication depends on verbal behavior—namely, the public sharing of similar reports on the results of similar procedures. We accept such shared reports as evidence for the recurrence of subjective experiences like ours in other minds because the brains and behavior of other adult humans are similar to our own. If you report seeing a circular gray afterimage surrounding the "b" in Figure 1, and I tell you that I see the same thing, each of us will ordinarily be willing to take the other's word for it without having other operational criteria for the occurrence of an afterimage, or a theory of how the operations of the eye and brain produce the effect. But we hesitate to make inferences about subjective experience in cases where we lack either underlying similarities of structure or verbal reports of experience (or both). The first case, where we have humanlike verbal behavior without brains or retinas, is the situation we confront in dealing with the issue of artificial intelligence. The converse case, in which we encounter humanlike brains and retinas but find no verbal behavior, is the situation we encounter in the question of animal consciousness.
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DOUBTS CONCERNING ANIMAL CONSCIOUSNESS |
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Doubts about the consciousness of animals occasionally surfaced even in classical antiquity. Augustine (City of God, 12.4) argued that nothing can be perceived without reason, which is needed to judge and classify sensations, and that beasts, lacking reason, therefore "lack understanding, sensation, and life altogether" (vel intellectu vel sensu vel vita omnino) (Green, 1966,
v. 4, pp. 16–17). However, pre-scientific philosophers, like most people, generally accepted without question the commonsense beliefs that a kicked dog feels pain and a repeatedly kicked dog suffers. The reality of animal consciousness was first called into serious question at the beginning of the scientific revolution by Descartes, who was also responsible for giving the phenomenon of consciousness its central place in the history of modern philosophy.
Descartes regarded animals as probably non-sentient automata. By Descartes' analysis, we perceive things consciously only because we are conscious that we perceive things (Malcolm, 1973). Since animals lack language, we can feel reasonably sure that they cannot reflect on their sensations; therefore, they cannot be aware of them. Descartes concluded that the beasts have sensations only in the sense that a mousetrap has a "sensation" of a mouse nibbling on its baited trigger. In both cases, there is stimulation of a sensor and a mechanical response, but there is no subjective awareness.
Later Cartesians accepted Descartes' view as a demonstrated fact. As Malebranche wrote in 1689:
In dogs, cats, and other animals, there is neither intelligence nor a spiritual soul in the usual sense. They eat without pleasure; they cry without pain; they believe without knowing it; they desire nothing; they know nothing; and if they act in what seems to be an intelligent and purposive manner, it is only because God has made them fit to survive, and has constructed their bodies in such a way that they can organically avoid—without knowing that they do so—everything that might destroy them and that they seem to fear. (Translated from Huxley, 1896
This view has persisted down to our own time, with natural selection replacing God as the agent of design. Perhaps the leading exponent of this position today is Daniel Dennett, who describes consciousness as an illusion from which non-human animals do not suffer. To quote Dennett (1999, pp. 292–293):
... What [our] early education produces in us is a sort of benign "user-illusion"—I call it the Cartesian Theater: the illusion that there is a place in our brains where the show goes on, towards which all perceptual "input" streams, and whence flow all "conscious intentions" to act and speak. I claim that other species—and human beings when they are newborn—simply are not beset by the illusion of the Cartesian Theater ... In order to be conscious—in order to be the sort of thing it is like something to be—it is necessary to have a certain sort of informational organization that endows that thing with a wide set of cognitive powers (such as the powers of reflection and re-representation). This ... is an organization that is swiftly achieved in one species, ours, and in no other.
Although Dennett is by no means alone in this belief, most of the scientists and philosophers who write about these issues regard our close animal relatives as having some sort of conscious mental life, but one which is defective and qualitatively inferior to our own. Most of these people think that language makes the crucial difference between human and animal consciousness, though they offer different reasons for thinking so.
Some insist that language is necessary for conceptual thought. Animals, they argue, can form no general concepts because they lack words. "Any thoughts [animals] express are particular, not general, and concern the present or future, not the past," contends Jonathan Bennett (1988, p. 204). "My hunch is that only through language can one show that one has thoughts that are not about what is present and particular." Speechless beasts are therefore compelled to think (if that is the correct term) wholly in terms of particular individuals, like someone condemned to speak a language consisting entirely of proper nouns (Pfungst, 1911; Adler, 1967).
One difficulty with this view is that many of our own concepts do not carry linguistic labels. Figure 2 presents three examples: an anatomical structure, an artifact, and a written symbol. Although anatomists, novelty salesmen, and typographers no doubt have names for these things, most of us have no idea what they are called. Nevertheless, we instantly recognize them as representing familiar classes of entities that we are accustomed to dealing with. We therefore have wordless concepts. Other animals may have them as well (Cartmill, 1990, 1996).
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Another supposed defect in animal mentation is often expressed by saying that beasts, lacking language, must of necessity lack self-awareness. Different writers make this claim in different words, implying that animals have primary consciousness but not higher-order consciousness (Edelman, 1989
), or that they have perceptual consciousness but not reflective consciousness (Griffin, 1992, p. 10–11), or creature consciousness but not state consciousness (Rosenthal, 1997), or phenomenal consciousness but not access consciousness (Block, 1995), or consciousness of qualia but not of information flow, or experiential sensitivity but not informational sensitivity (Flanagan, 1992, p. 147). What is asserted (roughly) by all these formulations is that although some nonhuman animals probably perceive things, none perceive themselves as perceiving them. Animals can feel (say) pain as an unconceptualized "raw feel" (Rorty, 1979, p. 24); however, they do not know that they are in pain, because they have no concept of "I" and no capacity for assessment of their own mental states. These abilities are supposedly conferred by the conceptual apparatus of language, which is necessary to allow self-representation—that is, thinking of one's self as an object. In Bickerton's words: What seems to us most striking about our kind of consciousness is its self-reflexive nature. We can perform a series of actions and at the same time observe ourselves performing them, so to speak ... This feeling of subjective consciousness is, of course, ‘What it is like to be a human’, and language contributes to it in a variety of ways. The most basic of these lies in providing the infrastructure for consciousness. You can't look at the spot you're standing on now if there is nowhere else for you to stand. A minimal prerequisite for self-consciousness is a place ... from which a part of you can look at another part of you. The secondary representational system [of language] is such a place. (Bickerton, 1990
Some psychologists and philosophers (e.g., Smith, 1986) claim or intimate that such reflexivity is a minimal condition for consciousness of any sort—that, in Rosenthal's (1993) words, consciousness is a reflexive feature of mental states, and that every conscious mental state must be in part about itself.
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THE DARWINIST'S DILEMMA |
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The opposite school of thought holds that language contributes little to consciousness, and that as Charles Darwin put it, "there is no fundamental difference between man and the higher mammals in their mental faculties" (Darwin, 1889, p. 66). This claim, which we will call anthropopsychism, is often condemned as unscientific, sentimental, or anthropomorphic. However, it has also been put forward by many scientific materialists as a rebuff to Christian or Cartesian dualisms that posit a profound spiritual difference between human beings and the beasts.
Darwin and many of his early followers held anthropopsychic views for just such strategic reasons. In the mid-19th century, the supposedly unbridgeable psychological and spiritual gap between people and beasts was generally perceived as an important objection to the thesis that human beings had evolved from apes. Darwinians accordingly labored to narrow that gap, both by exaggerating the humanlike characteristics of beasts and by promulgating racist stereotypes of "savages" as quasi-simian intermediates "bridging the psychological distance which separates the gorilla from the gentleman" (Romanes, 1889, p. 439).
Summarizing his own labors to that end, Darwin looked backward to the 18th-century vision of the Scala Naturae, laterally to the recapitulationism of von Baer and Haeckel, and forward to some of the central issues in 20th-century debates on these subjects:
... The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. We have seen that the senses and intuitions, the various emotions and faculties, such as love, memory, attention, curiosity, imitation, reason, &c., of which man boasts, may be found in an incipient, or even sometimes in a well-developed condition, in the lower animals ... If it could be proved that certain high mental powers, such as the formation of general concepts, self-consciousness &c., were absolutely peculiar to man, which seems extremely doubtful, it is not improbable that these qualities are merely the incidental results of other highly-advanced intellectual faculties; and these again mainly the result of the continued use of a perfect language. At what age does the new-born infant possess the power of abstraction, or become self-conscious, and reflect on its own existence? We cannot answer; nor can we answer in regard to the ascending organic scale ... That such evolution [of mental and moral faculties] is at least possible, ought not to be denied, for we daily see these faculties developing in every infant; and we may trace a perfect gradation from the mind of an utter idiot, lower than that of an animal low in the scale, to the mind of a Newton. (Darwin, 1889, pp. 126–127)
Darwin's theory of evolution by natural selection provides a model of how the mind of a Newton might evolve from that of an ape by imperceptible stages. Unfortunately, it also raises doubts about the significance of the expressive behaviors that Darwin pointed to as signs of "the various emotions and faculties ... in the lower animals."
To a consistent Darwinian, expressive behaviors demand explanations that extend beyond their proximate causes to their evolutionary origins. For example, grief is one proximate cause of human weeping; but this does not explain why we express grief in this particular way, or indeed why we express it at all. Why should we weep when we grieve? It seems counterproductive. (Why waste energy in sobbing, or lose body fluid by shedding tears?) To answer such questions in Darwinian terms, we must be able to attach some selective advantage to these expressive behaviors. But if a behavior's surface properties render it selectively advantageous, we can explain its origin and occurrence without reading any underlying mental events into it. We can reliably infer subjective pain from the yelp of a kicked dog only if the yelp serves no objective purpose. If it can be seen as adaptively useful—say, by serving to alert nearby kin to danger—then we can argue with Malebranche that God or natural selection has simply endowed dogs with a mechanical squeaker to enhance their reproductive fitness, and that "if they act in what seems to be an intelligent and purposive manner, it is only because God has made them fit to survive."
Darwinian anthropopsychism thus contained the seeds of its own denial. If expressive behaviors have no adaptive value, then natural selection cannot account for them. But if expressive behavior is adaptive, then it cannot furnish evidence for animal awareness.
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EPIPHENOMENALISM AND MORGAN'S CANON |
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Thomas Huxley evaded this dilemma by embracing epiphenomenalism—that is, the doctrine that the neural events that cause the behavior also cause the accompanying mental states, and that the mental events themselves are mere epiphenomena that have no effect on the behavior. As a Darwinian, Huxley was unwilling to believe that consciousness is a uniquely human property with no animal antecedents. However, published studies of the behavior of brain-damaged people convinced him that complex, even distinctively human, behavior is possible in the absence of conscious awareness. He concluded that in both beasts and people, consciousness is a functionless side effect of the true neurological causes of behavior. In Huxley's (1896
, pp. 240, 244) words: The consciousness of brutes would appear to be related to the mechanism of their body simply as a collateral product of its working, and to be as completely without any power of modifying that working as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery. Their volition, if they have any, is an emotion indicative of physical changes, not a cause of such changes ... [And] it seems to me that in men, as in brutes, there is no proof that any state of consciousness is the cause of change in the motion of the matter of the organism.
The most influential rejection of Darwinian anthropopsychism was offered by the British psychologist C. Lloyd Morgan, who in 1894 articulated the principle known as Morgan's Canon:
—In no case may we interpret an action as the outcome of the exercise of a higher psychical faculty, if it can be interpreted as the outcome of the exercise of one which stands lower in the psychological scale. (Morgan, 1977
Succeeding generations of experimental psychologists have adopted this dictum as a fundamental axiom in the study of animal behavior. Morgan's Canon is sometimes regarded as a special case of Occam's Razor, the principle that "It is vain to do with more what can be done with less" (Newbury, 1954; Sober, 1998; Staddon and Zanutto, 2000). If we can explain an animal's behavior as, say, a conditioned operant, then Morgan's Canon forbids us to interpret it as the outcome of the exercise of such "higher" faculties as volition or deliberation. Because Morgan's assumption places the burden of proof on the anthropopsychists in any argument about animal consciousness, it is perhaps the single most fundamental point at issue in our symposium.
Why should this point be at issue? Because it is not clear why Morgan's Canon is supposed to be parsimonious. On the face of it, it seems uneconomical to explain an animal's behavior in one way and a similar human behavior in a different way. Indeed, some argue that if we are going to invoke mental phenomena in accounting for our own actions, it is maximally parsimonious to explain other animals' behavior in similar terms whenever we can (Regan, 1983; Rollin, 1990; Bekoff and Allen, 1997).
Surprisingly, Morgan agreed that it would be simpler to do just that. He justified his Canon with reference not to the principle of parsimony, but to the fact of evolution, which he saw in characteristically 19th-century terms as the story of life's ascent through successive stages from the primordial ooze up to the lofty condition of man. The key to understanding Morgan's Canon is that phrase "lower in the psychological scale." For Morgan, "lower" did not mean "neurologically simpler." (A neurological criterion of simplicity would be of little use in practice, since in most cases the neurological causes of behavior are unknown.) Rather, "lower" for Morgan meant "historically prior," with specific reference to the story of human evolution. "Higher" here thus turns out to mean "distinctively human," and "lower" to mean "shared with other species." Therefore, the true, underlying implication of Morgan's Canon is that we are forbidden to interpret an animal's actions as the outcome of humanlike mental events, if we can find any other way of explaining them.
This reformulation shows us more clearly what is wrong with Morgan's Canon. The problem here comes into sharper focus if we transfer our attention from the brain to, say, the kidney. A urological version of Morgan's Canon would forbid us to interpret an animal's urine as the outcome of humanlike renal events—if we can find any other way of explaining it. If Morgan's Canon represents a parsimonious assumption, so does the urological version. But it seems obvious that the urological version is ridiculous. No physiologist would urge the adoption of such a rule to maximize parsimony and avoid the temptations of anthroporenalism.
On the face of it, Morgan's Canon has nothing to do with parsimony. Because humans are known to have conscious mental states, denying them to other animals saves nothing; it leaves us with the same number of entities on the bottom line of our overall ontological ledger (Newbury, 1954). If mental events sometimes cause human behavior, then positing different, non-mentalistic causes for similar behavior in other animals also saves nothing—unless we have independent reasons for thinking that those animals lack mental lives. And if we have such reasons, then Morgan's Canon is not needed to rule out mentalistic explanations of animal behavior.
Can Morgan's Canon be justified as a corollary of Occam's Razor? I think that it can, but that there is a high price to be paid—namely, accepting epiphenomenalism. If conscious states have no effect on human behavior, it becomes parsimonious to leave consciousness out of the picture when explaining similar behavior in other animals. Figure 3 makes this point in graphic form. Stimuli cause neurological events of which we are not aware. These events can cause behavior directly (as in a reflex), or cause other neurological events that are associated with conscious awareness. These conscious states may in turn produce chains of neurological effects that result in behavior. But if the dashed arrow in Figure 3 is wholly mythical—that is, if (in Huxley's words) states of consciousness never cause changes in the motion of the human body—then it does finally become parsimonious to simplify our models by denying consciousness in nonhuman animals. The parsimony interpretation of Morgan's Canon is therefore logically equivalent to Huxley's epiphenomenalism.
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This is not necessarily a defect. Epiphenomenalism is a respectable theory, backed up with powerful arguments by formidable proponents. As many of these proponents have noted, the fact that conscious states are correlated with our behavior does not imply that they cause it. If conscious states are physical phenomena, then we do not need to drag their subjective aspect into our explanations of behavior. And if they are not physical phenomena, then they cannot cause physical movement.
But epiphenomenalism also carries a high price tag. It does serious violence to our intuitive sense that our thoughts at least sometimes affect the things we do. And there is also a biological price to be paid for epiphenomenalism. If our conscious thoughts have no effect on our behavior, then they have no adaptive value—and so the evolution of consciousness cannot be accounted for in Darwinian terms (Allen and Bekoff, 1997, pp. 140–141).
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BEHAVIORAL CRITERIA OF CONSCIOUSNESS |
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Sober (1998)
offers to justify Morgan's Canon by reinterpreting Morgan's words "higher" and "lower" in terms of logical rather than evolutionary priority. He proposes that we should call one psychological faculty or internal mechanism "higher" than another "... if and only if the behavioral capacities entailed by the former properly include the behavioral capacities entailed by the latter" (Sober, 1998, p. 236). For any two alternative faculties or mechanisms A and B, if the behaviors permitted by A (A-behaviors) are a subset of those made possible by B (B-behaviors), then B is a "higher psychological faculty" in Sober's sense than A is. Accordingly, if an organism consistently demonstrates a capacity for A-behaviors but never displays B-behaviors, then we should not assume that its A-behaviors are produced by the B mechanism—even if there are other organisms in which the B mechanism produces A behaviors. This principle seems sound, but its applicability is much more restricted than that of Morgan's Canon. As Sober himself points out, psychological faculties are not always concentrically nested. They often partially overlap. For example, vision allows me to estimate an object's size, shape, and color, whereas touch allows me to estimate its size, shape, and temperature. It follows that neither vision nor touch is "higher" than the other by Sober's criterion, even though vision is a "higher psychological faculty" in the evolutionary sense in which Morgan's Canon was originally intended.
For similar reasons, mental events cannot be "higher" in Sober's sense than unconscious neural processes. If the epiphenomenalists are right, then unconscious neural processes cause all of our behavior, and are therefore as a class "higher" than mental events. Even if the epiphenomenalists are wrong, it is nevertheless clear that some behaviors (e.g., visual accommodation) result exclusively from unconscious neural processes. No matter what assumptions we make, the behaviors permitted by unconscious processes do not constitute a subset of those permitted by conscious mental events. Therefore, mental events cannot represent a "higher" internal mechanism (in Sober's sense) than unconscious neural processes. Sober's more sophisticated version of Morgan's Canon thus fails to justify the methodological assumption that nonhuman behavior should be attributed to unconscious or nonmental causes whenever possible.
The best reason for thinking that an animal lacks some psychological faculty is that it never exhibits behaviors that are uniquely associated with that faculty (Sober, 1998). To operationalize this principle with respect to animal consciousness, we need to know what behaviors are uniquely associated with consciousness. Unfortunately, comparative evidence is not useful here, since human beings are the only animals universally admitted to be conscious. However, since humans are not always conscious, we can at least ask what we can do when we are conscious that we cannot do when we are knocked out, anesthetized, or asleep.
This seems like an easy question to answer. As a rule, unconscious people are limp and passive and do not get up and walk around. When we are not sure whether a limp, passive person is conscious, we may run tests to see whether the person responds to stimuli in an intelligible and adaptive fashion (by answering questions coherently, pushing away the smelling salts, and so on). Our intuitive sense that most animals are aware of the world stems in part from the fact that they pass such tests by moving about and responding to non-verbal stimuli in an intelligible and adaptive fashion.
If these were decisive tests, we could tell whether an animal was conscious by pinching it. But these tests are not decisive, for at least two reasons. First, all animals respond adaptively to some stimuli; but most of them have nervous systems that seem too simple or diffuse (or even absent, as in the case of Protozoa) to be vehicles for conscious awareness. Second, people sometimes move about and respond to stimuli in a more or less adaptive fashion when they are not conscious—for example, in sleepwalking.
One can dispute whether sleepwalkers are unconscious. I have heard it argued that they must be conscious in some sense of the word, because they perceive and respond to the world and carry out complex tasks requiring the integration or "binding" of multiple sensory modalities. However, sleepwalkers usually claim that they have no awareness or recollection of what happens during their sleepwalking episodes. This claim is supported by electroencephalographic data. Sleepwalking usually begins during deep sleep (Stage 4) with slow-wave EEG rhythms. The EEG waves shift to higher frequencies during the sleepwalking episode. If the episode lasts 40 sec or less, the EEG resembles that seen in partial arousal from sleep; but longer-lasting episodes exhibit the EEG rhythms normally seen in light sleep and dreaming (REM) sleep (Jacobson et al., 1965; Gastaut and Broughton, 1965; Broughton, 1968; Crisp et al., 1990). A rarer form of sleepwalking that begins during REM sleep is sometimes distinguished as REM sleep disorder. Its associated behaviors are generally similar to those seen during typical sleepwalking with Stage 4 onset (Ishigooka et al., 1985). In cats, a syndrome resembling REM sleep disorder, in which locomotion, grooming, and even predation occurs during REM sleep (with alpha-rhythm EEGs), can be induced surgically by lesions of the nucleus ceruleus in the caudal brainstem (Sastre and Jouvet, 1979).
Typically, sleepwalkers open their eyes, sit up in bed with a blank facial expression, pluck aimlessly at the bedclothes, and then get up and walk. As a rule, they manage to move around without bumping into things, but they usually ignore the objects and people they encounter (Kales et al., 1966; Reite et al., 1990; Thorpy, 1990). Their actions are usually slow and aimless. More energetic and purposeful behaviors during sleep may include running, jumping, throwing things, driving automobiles, cursing, making simple responses to questions, and attacking bedmates and family members with fists or weapons (Yellowlees, 1878; Hartmann, 1983; Oswald and Evans, 1985; Bartholomew, 1986; Tarsh, 1986; Schenck et al., 1989; Mahowald et al., 1990). When forcibly awakened, sleepwalkers display intense anxiety, confusion, and no recollection of how they got where they are (Whitlock, 1975; Thorpy, 1990).
These facts suggest at least two general behavioral deficiencies associated with the absence of consciousness in humans. The first is a deficiency in social skills. Sleepwalkers typically ignore the people they encounter, and the rare interactions that occur are perfunctory and clumsy, or even violent. The other major deficit in sleepwalking behavior is linguistic. Most sleepwalkers respond to verbal stimuli with only grunts or monosyllables, or make no response at all (Pai, 1948; Gastaut and Broughton, 1965). Spontaneous utterances during sleep rarely exceed one or two words. More complex sentences or even conversations sometimes occur during the lighter stages of sleep (stage 1 non-REM sleep) at the beginning and end of the sleep cycle (Arkin, 1978), but these are uncommon and not associated with sleepwalking. In short, unconscious humans, like animals and very young children, are as a rule incapable of any verbal behavior beyond what Bickerton (1990) calls protolanguage: short, grammar-free utterances in which signs are used with referential meaning but syntax is absent.
These two apparent deficiencies may be significant. As noted above, it is sometimes claimed that syntactic language is a prerequisite for consciousness. This claim would be supported by a demonstration that linguistic behavior during sleep is restricted to protolanguage. And the deficiencies in social skills and behavior seen in sleepwalkers corroborate Humphrey's (1986, 1987) thesis that consciousness is a social adaptation, valuable chiefly because it facilitates the construction and perception of other minds. More detailed studies of the behavior of unconscious humans may provide decisive tests of these and other theories about the behavioral correlates of consciousness. If we can identify behaviors that are uniquely associated with consciousness in our own species, we may be able to draw some inferences about which animals lack it.
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SYNOPSISTHE PROBLEMS OF CONSCIOUSNESSDOUBTS CONCERNING ANIMAL...THE DARWINIST'S DILEMMAEPIPHENOMENALISM AND MORGAN'S...BEHAVIORAL CRITERIA OF...PROSPECTS AND CONCLUSIONReferences |
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From a scientific standpoint, the most important question to be asked about consciousness is "What brain mechanisms produce conscious awareness?" This question remains singularly refractory. But other questions about consciousness can be addressed without understanding its machineries; and because all these issues are logically interconnected, answering any of those other questions will contribute to an eventual solution of the central problem. If we had a physical account of the mechanisms of consciousness, we could determine its distribution in other animals and study its evolution. Conversely, given a reliable behavioral test for the presence of consciousness in other animals, we could start mapping its distribution and looking for similarly distributed neurological correlates to help us understand its mechanisms. If we knew something about the distribution and behavioral correlates of consciousness, we could utilize the comparative method to investigate its adaptive value. If consciousness has adaptive value, then philosophical epiphenomenalism is a mistake. Once this knot is loosened at any one of these points, it will begin to come apart everywhere.
These are not unresolvable questions. Scientific inquiry has already yielded information about the presence or absence of consciousness in nonhuman organisms. It seems clear that organisms that lack a nervous system are unconscious Cartesian automata. There are empirical reasons for thinking that this may also be true of most other organisms. On the other hand, there is growing evidence, much of which is summarized by the other contributors to this symposium, that some non-human vertebrates have cognitive, social, and linguistic abilities that resemble our own in various ways. That accumulating body of evidence refutes certain claims about the uniqueness of human mentation. Although the problems of animal consciousness are not likely to be resolved completely until the mechanisms of human consciousness are understood, we know some relevant facts. We can reasonably expect to know more in the future if we avoid the temptations of skepticism.
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1 From the Symposium Animal Consciousness: Historical, Theoretical, and Empirical Perspectives presented at the Annual Meeting of the Society for Integrative and comparative Biology, 6–10 January 1999, at Denver, Colorado.
2 E-mail: matt_cartmill{at}baa.mc.duke.edu
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