In the “Apology for Raymond Sebond” Michel de Montaigne aims to upend human arrogance about our superiority over other animals by enumerating the many wonders of the nonhuman animal world. We see birds building intricate nests; spiders thickening and slackening their webs with an engineer’s precision; ants and bees organizing themselves into complicated societies; and dogs making reasoned judgments about the likely location of their prey. Although we may be inclined to dismiss these behaviors as mere instinct, Montaigne argues that it is presumptuous “to judge that the beasts do by natural and obligatory instinct the same things that we do by our choice and cleverness.”Footnote ¹ And even if other creatures do act with less conscious design than we do, he asks, does that make their actions any less wondrous?

Nussbaum’s book has much in common with Montaigne’s argument. She too highlights the many wondrous behaviors of nonhuman animals to subvert human hubris and elicit greater concern for them. Wonder is one of three emotions—the other two being compassion and outrage—that she identifies for motivating people to care, or care more, about animals (8). The word “wonder” appears, by my informal count, close to 50 times in her book, with “wonderful” and “wonder-inspiring” appearing another 10 times. One of the great pleasures of writing her book, she notes, was the opportunity it provided to immerse herself in the detailed study of animal lives (xiv), and she offers many detailed accounts of the wondrous lives of many different animals throughout it. She repeats the same story as Montaigne, drawn from Sextus Empiricus, about a dog who uses logic to determine the location of its prey (23–24).

Nussbaum’s earlier work cites Aristotle to the effect that “there is something wonderful and wonder-inspiring in all the complex forms of life in nature.”Footnote ² This book develops this sentiment at length. She describes wonder as a combination of awe and curiosity inspired by our perception of other creatures’ striving for significant goods. Her sense of wonder infectiously pervades this book and distinguishes it from her other work on capabilities and animals. The lack of wonder is, conversely, one of the major flaws Nussbaum finds with other major approaches to animal justice. The “So Like Us” approach, which makes the case for extending moral consideration to nonhuman animals based on their similarity to us, lacks “wonder at the diversity of nature, love of its many distinctive forms of life” (33, cf. 37–38). Utilitarianism is similarly deficient “in wonder” as it bases animal’s moral value on the “least common denominator view” of their capacity for pleasure and pain (41). Christine Korsgaard clings to the Kantian idea that human moral rationality elevates us above other animals, and consequently, fails to appreciate that “all lives are uniquely wonderful in their own way” (78).

Nussbaum argues that the capabilities approach is the ethical and political framework best suited for respecting the wondrous diversity of animal lives. It rests on the intuition that it is wrong to needlessly impede or thwart the striving of creatures toward significant goods that characterize a flourishing life for them. It exhorts us to explore their diverse types of flourishing so that we can stop our wrongful interference with them and positively support them when possible—for example, by protecting habitats.

Although Nussbaum shares with Montaigne a sense of wonder about animals, she is more bullish about the powers of reason to distinguish truly wonderous creatures from others. Midway through her book, in Chapter 6, she sets down theoretical criteria for drawing a line between sentient creatures, who matter for justice, and non-sentient ones, who do not. Nussbaum indicates early in her book that she will make this move but it is still jarring because in earlier chapters she criticizes the scala naturae idea and Korsgaard for elevating humans above other animals. Here she follows utilitarian theorists in doing much the same, albeit by elevating all sentient creatures (not just humans) above non-sentient ones. “Here we come back to the great truth in Utilitarianism: there is a dividing line in nature created by sentience” (138). She defines sentience broadly to include not just the ability to feel pleasure and pain but also having a subjective point of view, cognitive awareness of objects, and movement toward the good and away from the bad (137–38). Even so, she puts sentient creatures on a pedestal much as others do with humans (57).

Nussbaum is liberal in her judgment about which animals should be placed on the sentience side of nature’s line: all mammals, all birds, all bony fish, cephalopods, probably reptiles, and probably crustaceans. Only elasmobranch fish, including sharks and stingrays, insects (with the possible exception of bees), Cnidarians (corals, jellyfish, sea anemones), Porifera (sponges), and plants are left out. Her judgments on sentience are humble and disputable, as new knowledge may lead to new conclusions. “The theory is the important thing” (120).

It is the theory that I mean to question. It is not clear to me why Nussbaum thinks line-drawing around sentience is necessary for the capabilities approach. She argues sensibly enough that the capabilities approach requires some evidence of striving. If a being cannot be perceived as striving for something, then it cannot be said to flourish and the capabilities approach does not apply. Yet, sharks, spiders, sunflowers, and trees all demonstrate striving toward types of flourishing (for example, the acorn “strives” to grow into an oak tree). Why must they feel or have a subjective point of view to command our respect? Why is their striving—and our wonder at it—not enough? Nussbaum suggests several possible answers. Automata do not require consideration under the capabilities approach because they do not strive (127). There is no harm in stopping a child’s wind-up toy from racing off the table’s edge because it has no ends or flourishing. She argues that insects and plants are automata in this same way. Yet, they all have ends and react to their environments in ways that wind-up toys do not. Moreover, as Montaigne points out, even if these beings are driven largely by natural instincts, that does not make them any less wondrous.

Nussbaum argues that plants, at least, cannot be accommodated within the capabilities approach because they are cluster entities lacking in individual life, death, and flourishing (151). I am doubtful about the veracity of this claim when applied to many plants but even if it is true of some of them—say a cluster of aspen trees (e.g., Pando)—can we not just treat that entity as an individual under the capabilities approach?

Nussbaum’s concern with line-drawing might follow from practical considerations. The purpose of her book is to change the way humans think about and treat animals (xvi). It concludes by offering practical suggestions for changes to laws that would improve most animal lives. Extending the capabilities approach to include justice for all beings would complicate this purpose. It is understandable, then, if she chooses for practical reasons to draw the line at sentience in arguing for justice for animals. But this would make her line-drawing exercise not part of her theory but only a strategic application of it.

The capabilities approach seems to be able to encompass all living things that display significant striving without requiring any further proof of their subjective experience. In determining whether to extend respect to any living being under the capabilities approach, rather than ask “Can they suffer?” or “Do they have subjective experience?” we might ask “Do they show purposeful striving?” If they do, we might extend some rights to them, whether they be dolphins, spiders, or trees. The need for something like a human-type “will” seems unnecessary for discerning purpose and striving.

Nussbaum’s logical line-drawing felt to me not only out of character with the wonder that animated the rest of her book but also potentially counterproductive. If the striving of sharks, spiders, and sunflowers do not count under the capabilities approach, one might be more inclined to doubt the relevance of the striving of tuna and chickens. The precise sort of line-drawing that Nussbaum engages in undermines the wonder that she relies on to motivate concern for nonhuman animal lives in the first place. Wondering at the lives of all living things and working toward a world where we respect all their diverse capabilities raises some thorny questions. We must eat and build homes and doing so invariably pits our flourishing against the flourishing of other living beings. Nussbaum lays the groundwork for working through these sorts of moral dilemmas, however, by defending a self-defense and personal health principle (Chapters 7–8). Extending these principles might show how we can reasonably respect the capabilities of all living striving beings without sacrificing ourselves—and importantly, without drawing lines around sentience. While Nussbaum does not push the capabilities approach this far, it seems a viable next step.