Animal sensitivity from a philosophical perspective: from ontology to ethics

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Since antiquity, the question of whether animals feel and what this implies for our behavior has permeated the history of philosophy. Broadly speaking, the focus has oscillated between ontological debates about the nature of sentience and moral discussions about the weight of suffering versus other supposed markers of dignity, such as rationality. In the last 150 years, interest has shifted notably toward the ethical, prioritizing harm and well-being over old hierarchies. Today, the guiding principle is clear: if there is the capacity to feel, there are grounds for moral consideration.

It wasn't always this way. Much of the Western philosophical canon minimized or reinterpreted animal experience in ways that justified its use and domination. Leading figures from Plato to Descartes and Kant offered arguments—some metaphysical, others normative—that placed humans at the center. However, the contemporary debate has taken a turn: it now addresses speciesism, rights, and welfare; it reclaims notions such as the subject of a life and denounces the willful ignorance that makes the unbearable bearable. Between philosophical critique and social pressure, a powerful idea emerges: integrating other animals into the sphere of morality and positive law.

What we mean by animal sensitivity and why it matters

In simple terms, animal sentience refers to the capacity to experience pleasure and pain, fear, relief, joy, or other conscious experiences. This is not merely a theoretical matter: for much of contemporary ethics, the recognition of this capacity implies the obligation to take animals seriously. The fundamental question was canonically posed by a line of thought that runs from Bentham to current debates: it is not so much whether they can reason , but whether they can suffer. When an individual can suffer, their suffering matters morally.

However, pain and suffering are not uniform. In humans, the experience is expressed linguistically and complicated by the anticipation of future harm; in many non-human mammals, the experience can become indiscriminate in the face of threats, so that their panic is even more intense in certain situations. Not having articulate language does not equate to not suffering , and the absence of obvious signs is not proof of insensitivity.

There is, however, a gradation linked to the complexity of the nervous system. It is unreasonable to attribute to an earthworm or a sponge the same repertoire of ailments as to a mammal. This point has led to linking relevant rights or protections with sentience, modulating the scope of obligations according to sentient capacities. In short, sentience functions as a practical criterion for guiding duties.

From Antiquity to Christianity: Pythagoras, Plato and the Biblical Tradition

The figure of Pythagoras is often invoked as an emblem of compassion for animals, but the classical reasons differ from current arguments. The famous anecdote recounted by Diogenes Laertius, in which the sage stops someone whipping a dog because he recognizes in its howls the voice of a deceased friend, reflects a belief in the transmigration of souls rather than a defense of animals per se. The concern, in this case, stems from the fear of harming a reincarnated human intelligence . Furthermore, there is evidence that questions the extent to which Pythagoras always opposed sacrifice: it has been said that he celebrated a mathematical discovery with the sacrifice of oxen, a story present in contemporary scholarly discussions.

In Plato's case, the matter is even more slippery. The Athenian's supposed vegetarian diet is difficult to prove historically, and even granting it, its philosophical foundations do not translate into duties toward non-humans. Plato imagines a Golden Age in which the consumption of animals would be unnecessary, but our world does not belong to that mythical era; therefore, such a regime would not be required under ordinary circumstances. More importantly, he defends the superiority of the human soul, immortal and rational, and thereby legitimizes the use of animals for human purposes. The Platonic ideal does not crystallize into moral obligations toward other species.

The influence of these ideas reappears in Christian authors who shaped the Middle Ages. Genesis evokes an initial time of harmony between humans and animals, but after the flood, Noah and his descendants are given explicit permission to consume every living thing. This line of thought, nuanced in many ways, leads to thinkers like Augustine and Thomas Aquinas, who, through different paths, uphold the legitimate dominion of humans and deny the existence of direct duties of justice toward animals. In this tradition, only human interests occupy the center of creation and of the law.

Modern turn: Descartes and the animal machine

With modernity, far from softening, the divide was reformulated in language befitting the new science. Descartes, a key figure, proposed a sharp distinction between res cogitans and res extensa, placing animals on the side of machines. Following this line of thought, non-humans would be thoughtless automatons, and this has been interpreted as denying them pain and pleasure. Although some contemporary interpretations qualify this conclusion, what matters for practical consequences is that, within his own framework, there would be no compelling reasons to restrict practices such as eating animals or the then-nascent vivisection. The image of the animal-machine served as a theoretical pretext for normalizing cruelty.

Kant and the relative value of the non-rational

When discussing modern ethics, few names carry as much weight as Kant. However, his treatment of the moral status of animals seems uninspiring from today's perspective. For the philosopher from Königsberg, rationality is the foundation of normativity: only where there is reason are there duties properly speaking. Thus, non-rational beings have a merely relative value, comparable to that of objects, and are not ends in themselves. Despite this, Kant recommends avoiding cruelty, not because animals have rights, but as an indirect duty toward ourselves: violence against them dulls our moral dispositions toward other humans. Compassion for animals, yes; recognizing their rights, no: that is the Kantian limit.

From speciesism to moral consideration: contemporary critiques and proposals

Much of recent philosophy diagnoses a pervasive bias: speciesism. The term, popularized by Peter Singer and rooted in the earlier work of Richard D. Ryder, names discrimination based on species, analogous to racism or sexism. The core of the critique argues that giving more weight to the interests of our species simply because we belong to it is a prejudice. The yardstick should be the capacity to feel, not species identity.

Speciesism, moreover, feeds on social inertia. From childhood, a saccharine image of the animal world is normalized, where happy beings "must" die for our consumption without it seeming reprehensible. Selective discrimination also occurs: emblematic, large, or charismatic species are fervently protected, while the suffering of others, equally or even more sensitive, is ignored. Added to this are religious and superstitious factors that shape affections and contempt. All of this violates the principle of impartiality, since what is wrong to do to a human should not be relativized when the victim is non-human.

In this context, the idea of ​​willful ignorance is key. Many people prefer not to know how certain products reach our tables or what is done in certain laboratories, to avoid psychological discomfort. The media often contributes to this veil. This chosen blindness, as broader historical episodes show, deteriorates the quality of democratic life. Refusing to see does not absolve us of responsibility; in fact, it often exacerbates it.

So, how can we positively reformulate the moral framework? An influential formulation in animal ethics comes from William Frankena: all beings capable of conscious experience deserve consideration in their own right, insofar as our actions affect them. This perspective connects with Bentham's famous criterion and with contemporary utilitarian positions. The threshold that matters is sentience, not perfect rationality.

Another crucial contribution comes from Tom Regan, who coined the notion of the subject of a life to ground rights. An individual is a subject of a life if they possess a biography with perceptions, desires, memory, interests, and a certain sense of the future. From this, Regan argues that such subjects have rights that do not depend on the reciprocity of duties or their belonging to the human species, though he does qualify the scope: in its original version, the category applies primarily to mammals and, more strictly, to mammals of a certain age. This underlying thesis challenges Rawls's idea that there are no rights without duties and engages with the notion of interests championed by Joel Feinberg.

Alongside this, Christine M. Korsgaard has reframed the issue from the perspective of autonomy and practical norms, arguing that our own sources of obligation bind us to other animals. These positions coexist with a terminological and legal debate: speaking of rights can refer to moral rights or legal rights, with distinct statuses, and it is not trivial whether a chimpanzee is granted human rights or rights as a chimpanzee. The label matters less than the recognition that they are not mere means.

It is also important to distinguish between animal rights advocates and animal welfare advocates. The former, of an abolitionist bent, seek to end the exploitation of animals as tools, relying on both deontological and utilitarian arguments. Welfarism, on the other hand, is a reformist stance that advocates for reducing suffering deemed unnecessary, while acknowledging that some harm may be considered useful in certain circumstances. Since 1979, this approach has developed a well-known framework: the Five Freedoms, promoted by the work of the British Farm Animal Welfare Council, which are applicable in principle to all animals under human care. Welfare is defined negatively by the absence of harm and positively by allowing natural behaviors.

1. Without hunger or thirst: continuous access to adequate food and water.
2. Without discomfort: appropriate environmental and shelter conditions.
3. Without pain: prevention and relief of avoidable suffering.
4. Without wounds or diseases: health care and prevention.
5. Without fear or stress: management that minimizes panic and anxiety; and positive freedom to express species-appropriate behaviors.

Phenomenology and deconstruction: Levinas and Derrida on animal otherness

In the phenomenological tradition and its critique, the relationship with otherness has been examined from angles that directly address the animal question. Levinas denounced the primacy of the theoretical attitude, that gaze which reduces the other to the same and subordinates otherness to the subject's dominion. In contrast, he described an original ethical relationship in which the other appears with a face that compels us. What is striking, for our purposes, is that this otherness manifests itself to consciousness as eminently sensory.

Derrida takes up and challenges this legacy. On the one hand, he deconstructs modern subjectivity by revealing its exclusions; on the other, he questions whether the term "human" adequately defines the field of otherness. According to his critique, Levinas's humanism risks excluding non-human animals by definition. Derride's proposal, therefore, is to consider animality as the common substrate of all otherness, shifting the boundary that separates the human from the non-human. Otherness is not the exclusive property of humans; animal sensibility overflows that threshold.

Between sensitivity and law: social practices, consumption and institutional change

No one is surprised anymore by the revulsion provoked by images of violence against animals; and yet, we live with production systems that multiply suffering and death by millions. This paradox is partly explained by the gap between what we deem right and our habits, by the incentive structure that makes it easy for us to look the other way, and by historical inertia that is slow to change. Psychology, sociology, and history help us understand the slowness with which collective practices change. The dissonance between moral convictions and consumption habits is as real as it is unsettling.

Hence, many voices insist on the crucial step: translating moral obligations into legal norms and applying them effectively. Social awareness is not enough; institutional recognition is required to guarantee minimum standards of treatment and minimize suffering. Whether viewed through the lens of rights or robust welfare standards, the practical challenge lies in designing rules, monitoring their enforcement, and penalizing their violation. When the law embodies moral considerations, improvements cease to depend on individual goodwill.

Key works and references

• On Cartesianism and animals: contemporary analysis of the animal-machine thesis, with works such as that of Peter Harrison that review interpretations and practical consequences.
• Classical tradition: testimonies about Diogenes Laertius and recent readings of Pythagoreanism (e.g., in the Stanford Encyclopedia of Philosophy), as well as debates on vegetarianism in Plato and its doctrinal coherence.
• Contemporary ethics: Peter singer and his critique of speciesism in Animal Liberation; Christine M. Korsgaard and the grounding of our obligations to other animals in Fellow Creatures.
• Rights based on the subject of a life: proposal of Tom Regan, in dialogue with the idea of ​​interests of Joel Feinberg and with Rawlsian objections regarding rights and duties.
• Biblical and theological framework: passages from the Genesis that inform readings on human domain, and its reception in Agustín y Thomas Aquinas.

The picture that emerges from this journey is clear: from ancient and medieval interpretations that justified a dominion without direct obligations, through modern rationalism that technologized distance, to a contemporary moral philosophy that places suffering and sentience at its center, the debate has been refining its compass. Today we have critical concepts like speciesism, criteria like the subject of a life, and practical frameworks like the five freedoms; we also have a profound examination of otherness that questions human exceptionalism. All of this points to a shared task: reducing avoidable suffering, recognizing other animals as recipients of our obligations, and transforming that conviction into practices and laws commensurate with what we claim to value. It is not just about thinking better, but about living in accordance with that improved reflection.