Humans have a long-standing fascination with how other animals might see the world. Our curiosity about our own mental lives extends easily to other species, especially when their actions are similar to our own. The perennial question, of course, is whether our perception of a mind in another species is simply a projection of our own thoughts and feelings or whether we are perceiving something that truly exists.

For René Descartes, the answer was notoriously simple: Lacking souls, nonhuman animals were mere automatons with no reason, rationality, or mental life. (It should be noted that Descartes was writing at a time when clockwork mechanisms had only just been invented, and there is a sense in which his comparison was meant to invite wonder and awe—not to denigrate, as we assume now.) Darwin rejected this Cartesian assessment, arguing that the difference between humans and other species was one of degree, not of kind. In The Descent of Man (1871), Darwin applied his theory of evolution explicitly to humans, tracing the origins of human behavior and psychology to our primate ancestors and beyond. This stance was taken further by George Romanes, Darwin's friend and protegé, who argued that it was possible to use an introspective assessment of one's own mind to infer the mental states of other animals by using a method called double induction. The rise of behaviorism in the early twentieth century brought another pendulum swing, and the attribution of invisible mental states and emotions to animals was considered scientifically dubious, if not outright heresy. As the end of the twentieth century approached, a reaction against behaviorism gained momentum, heralded by Donald Griffin's book The Question of Animal Awareness (1976), in which the mental life of animals was unabashedly reintroduced to comparative psychology by direct analogy with our own. Now, at the beginning of the twenty-first century, studies of the human-like nature of animal cognition have become something of a boom industry, with demonstrations of everything from grief in chimpanzees (Anderson et al. 2010) to charades in orangutans (Cartmill and Byrne 2007) to “counterespionage” in scrub-jays (Dally et al. 2010).

Recent books by Jonathan Balcombe and Dale Peterson capture the Zeitgeist superbly. Balcombe's Second Nature: The Inner Lives of Animals and Peterson's The Moral Lives of Animals both use the burgeoning scientific literature on animal minds to argue that human arrogance about our intelligence and superiority is misplaced. In summary, they suggest that our increasing knowledge of animal cognitive and emotional complexity requires that we rethink our attitudes about animal welfare and reconsider the manner in which we exploit the Earth's resources. It is apparent in both volumes that the authors offer polemics: Neither attempt to present a balanced view of the literature, nor do they offer any critical assessment of the available evidence.

Taken on their own terms, these books achieve their stated goals admirably and use a similar structure to do so. Both authors present comprehensive reviews of the evidence for the sophistication of animal thought and feeling in the first two-thirds of their books and then deliver an overt political message in the final section. Both authors skillfully weave empirical scientific findings into their own ideologies to produce books that are engaging and thought provoking. The degree to which we are convinced by their arguments, however, depends on our previous familiarity with the animal-cognition literature and how much we have deliberated on the philosophical issue of what constitutes ethical behavior. From my perspective as a zoologist and psychologist, I found plenty to dispute in the authors' cherry-picking of literature and one-sided presentations, but I couldn't find much that changed the way I think. The findings from the world of animal behavior and comparative cognition are not as cut and dried as the authors suggest, and consequently, the issues are more complex than they are made to appear.

In The Moral Lives of Animals, the argument is tightly focused on whether animals besides ourselves lead “moral lives.” Peterson's answer is that they do. He leans heavily on Darwin's argument for differences in degree only and relies on Darwin's ideas concerning the link between “social instincts” and the development of a moral sense. In this, Peterson is not alone. In most comparative-cognition studies, precisely this justification is used: Our own highly sophisticated abilities did not, it is argued, spring forth fully formed, like Athena from the head of Zeus, but instead represent the accumulated changes brought about by the action of natural selection. It should be possible, therefore, to detect either the same abilities (where they are shared by descent) or their precursors (where they are built on and refined with the use of language and culture) in other animals.

For the most part, Peterson uses vivid and effective examples to illustrate his points, although whenever he confronts a particularly tricky issue, he resorts to describing his pet dogs' behavior, making it very hard to assess or dispute his arguments; we basically have to take his word for it. He does, however, avoid the more obvious trap of anthropomorphically projecting human values onto other species, and he takes pains to explain why such a position is false. He also makes it clear that, sometimes, the behaviors displayed by other species can be explained by simple learning mechanisms.

To make his argument about the evolved nature of moral sentiments stick, however, Peterson has to make the further claim that human morality is not the vexed issue that has plagued philosophers and theologians for centuries but, instead, can be characterized as a set of “unspoken and unwritten rules of urge, inclination, and inhibition” (p. 82) that exist across the animal kingdom as a whole. That is, although “language can help reveal the invisible structures of human morality” (p. 82), it is not, in any sense, constitutive of our morality. This kind of argument reduces morality to a set of instincts and offers a functional definition that is sufficiently broad to encompass a wide variety of other species, allowing Peterson to chip away at the notion of human exceptionalism or, as he calls it, our “Darwinian narcissism.”

But what if you don't agree with this definition? What if you happen to think that language does more than simply allow us to debate morality, that it actually helps to generate morality in the first place? What if you accept, for example, John Searle's argument in The Construction of Social Reality (1995) that human reality is partly constituted by “institutional facts”? Searle states that many of our “facts” about the world do not refer to an objective physical reality but are created by a collective agreement of the people living in a culture. As such, these institutional facts are heavily dependent on language and representational thought. Money, government, and marriage are institutional facts; we reflect on these in ways that have great moral significance. Deciding on what constitutes moral truth, then, is “not a solitary pursuit, but a public endeavor,” as Michael Sandel argued in his book Justice: What's the Right Thing To Do? (2009, p. 28).

To his credit, Peterson does raise the issue of cultural and historical variability of moral values. (Slavery and female suffrage are both mentioned explicitly in this context.) He argues, however, that these are “surface phenomena” and that the “deep” structure of human morality—the negotiation of inherent conflicts and our understanding of things as good or bad—remains intact. But I'm not sure it's that easy. The empathic, instinctual, nonlinguistic root of morality that Peterson defends cannot deal with the more complex aspects of human life. Although it seems perfectly plausible that our emotional responses to issues concerning the fairness and justice of particular practices reflect the kind of social instincts identified by Peterson (and indeed by Darwin before him), such responses do not exhaust what it means to be a moral being in human society.

Moral questions are often hard to evaluate using only a “gut instinct,” because more than one moral principle can be invoked: Should we choose the option that maximizes overall well-being? Should we respect individual freedom? Should we promote virtue? All moral principles do not conform to the utilitarian stance that Peterson accepts as axiomatic. This, then, is more than a simple argument over surface qualities or the recognition that moral values can change over time. It is about making complicated decisions about our behavior using the institutional facts that organize our lives in fundamental and far-reaching ways.

As yet, we have no evidence to suggest that any nonhuman animal is capable of forming institutional facts, and this may reflect a similar lack of unequivocal evidence to show that nonhumans possess the ability to comprehend unobservable abstract entities—notably, the ability to have thoughts about their own and others' thoughts. Peterson simply does not address the debate surrounding the metarepresentational capacities of other animal species, however essential to his argument it would seem. This is why the degree of satisfaction that we derive from reading The Moral Lives of Animals rests on how familiar we are with the relevant literature. The diminishing of human morality succeeds only to the extent that we remain convinced that there is nothing exceptional about human language and cultural abilities in relation to generating moral values. This, in turn, succeeds only in so far as we know (or don't know) about the relevant research in animal psychology and philosophy that offers an alternative view to the one that Peterson presents.

In Second Nature, Jonathan Balcombe also avoids the obvious trap of simple anthropomorphic projection, but Balcombe does this by introducing von Uexküll's concept of the umwelt, the perceptual world of an organism. Given that other organisms possess different kinds of sensory apparatus and morphology, they are likely to experience the world in ways very different from those in which humans do, and we cannot assume that their view of the world maps onto ours congruently. Balcombe illustrates this concept effectively, and unlike Peterson, demonstrates less of a tendency to undermine human abilities in an attempt to level the playing field—although the temptation to do so is clearly strong: “Humbling as it may be, for all our vaunted brain power, humans emerge as nothing special in the sensory sweepstakes. Our senses of vision, hearing, smell, taste, and touch are middling at best” (p. 15). The problem with a point like this one is that humans make good use of all of these senses, unlike many other species, which gives us a broader umwelt, a greater sensitivity to our environment, and, therefore, a greater flexibility in our perceptions of it.

To make his case for the emotional complexity of nonhuman animals, Balcombe emphasizes the extent to which other animals' lives are as rich and fulfilling as those of humans and are possessed of exactly the same kinds of conscious experience, albeit a consciousness reflective of that species' umwelt. In many ways, this is a more interesting approach. The concept of the umwelt is one that needs greater prominence in the study of animal behavior and psychology, and the emphasis placed on animals in their own worlds is exactly as it should be. Despite this, I found Balcombe's approach to be suffused with a creeping anthropocentrism. His overarching aim in Second Nature to “close this gap between the human beings and animals—by helping us understand the animal experience, and by elevating animals from their lowly status” (p. 4) has the effect of ensuring that animal traits remain aligned to a human standard. I have no doubt that Balcombe would disagree with this assessment; nevertheless, his frequent insistence that the abilities of other animals are not just different from those of humans but often superior has precisely this effect. Humans are constantly pulled into the comparison, causing the book to maintain a heavily anthropocentric tone, even as it strives to undermine anthropocentrism. It is only as we embark on the final section of the book that the need for this particular stance becomes clear: Balcombe drives home the point that if animals experience their lives much as we experience our own, our treatment of them in ways that are self-serving is morally and ethically unsound.

Scientifically, the aim is to discover what is actually going on, not simply to advocate for one viewpoint or another. If we cannot simply assert that other species lack feelings or a conscious awareness of their own lives, then, surely, the same must be true for those who wish to assert otherwise.

Despite possessing lovely turns of phrase (e.g., “the bat cyclone suddenly pours forth, like tea from a spout,” p. 11), Second Nature mostly consists of the relentless piling on of examples that demonstrate how smart, empathic, compassionate, and cooperative other animals are in their various ways. Balcombe also displays an interesting tendency to swerve into unsupported assertions at crucial points: “The timing and tenor of each bird's movements were those of a conscious, flexible being. The raven's day, like the gull's, is lived consciously” (p. 62). The question of what exactly is meant by “consciously” in this context (and indeed throughout the book) is left open, as is the means by which one identifies consciousness through observation of behavior alone. Similarly, when reporting on young elephants orphaned by poaching and culling, Balcombe suggests that they manifest symptoms of post-traumatic stress disorder, “including what appear to be nightmares” (p. 59), but he leaves unanswered the question of how to determine whether an elephant is reexperiencing a past event. Balcombe confidently asserts elsewhere that “there really can no longer be any legitimate doubt that great apes, our fellow primates, possess a degree of awareness on par with our own” (p. 63). Ironically, this statement is at odds with the ongoing (and often heated) debate on this matter (Call and Tomasello 2008, Penn and Povinelli 2007).

More so than with The Moral Lives of Animals, Second Nature lacks critical assessment of any kind, and the book offers no consideration of alternative viewpoints. Balcombe wants to argue that the burden of proof falls on those who would deny animals their thoughts and feelings, but this is not how it works. Instead, the burden of proof falls squarely on all of us, all of the time. Scientifically, the aim is to discover what is actually going on, not simply to advocate for one viewpoint or another. If we cannot simply assert that other species lack feelings or a conscious awareness of their own lives, then, surely, the same must be true for those who wish to assert otherwise.

Interestingly, amid all their talk of evolutionary continuity, both Peterson and Balcombe display a tendency to remove humanity from nature. They speak of humans largely in association with the disruption and destruction of other species; humans impose on the natural world, but somehow we do not inhabit it in the manner of other species. That's one way to look at things, of course, but another way is to consider that we are always and forever a part of nature, just like all other species. We do not, and cannot, sit outside the natural world. Yes, we can transform, modify, and control our environments, including those of other species, in ways that are unprecedented in the history of life on Earth, but this does not remove us from nature. Instead, it is—for better or worse—the human adaptation to the natural world.

The differences between humans and other species lie in our reflexivity with respect to our actions: We can see and predict the consequences of our behaviors, we understand how our actions influence and affect the lives of other species, and we can recognize (and even hold) more than one moral position pertaining to those actions. These distinctions must matter in any comparison among species, and glossing over them—either by diminishing human morality or by elevating the abilities of other species—does not make them irrelevant. Revealing the complexity and subtlety of animal behavior is a laudable and admirable goal and, in both books, the authors' love and appreciation for other species shines through to great effect. To achieve this goal without recognizing the peculiar nature of human life on Earth and without accepting that there are very real differences that make a very real difference seems like an opportunity missed.