Aristotle grouped living things primarily by their observable traits, their modes of reproduction, and what he saw as their capacities of soul: the ability to grow, to sense, and to reason. Writing in the fourth century BCE, he produced what amounts to the first large-scale biological survey in Western history, cataloguing roughly 500 animal species across several treatises and arranging them into nested groups based on shared physical features, habitats, and behaviors. His system was not identical to modern taxonomy, but the impulse behind it, sorting organisms by structural similarities rather than by myth or culinary use, was strikingly modern for its time.
The Soul as an Organizing Framework
At the highest level, Aristotle separated all living things by what he called the functions of the soul. Plants possessed only a vegetative soul, responsible for nutrition, growth, and reproduction. Animals possessed a sensitive soul on top of the vegetative one, giving them the ability to perceive the world and, in most cases, to move. Humans had all of these plus a rational soul, the capacity for thought and deliberation. This layered scheme meant every organism shared the most basic life functions with every other organism, and more complex creatures simply added new capacities on top.
The vegetative functions were, for Aristotle, the foundation of all biological explanation. Because every living thing must nourish itself and reproduce, these activities came first in any account of why an organism is built the way it is. An animal’s organs exist to serve its survival and reproduction, and everything else, perception, locomotion, even intelligence, builds on that base.1Aristotle on Teleology. Teleology and Organisms i: General Principles This idea that an organism’s parts exist “for the sake of” the whole creature and its way of life is what scholars call Aristotle’s teleological approach to biology: form follows function, and the function that matters most is staying alive long enough to produce offspring.
Blooded Versus Bloodless
Within the animal kingdom, Aristotle drew a major line between creatures with blood and creatures without it. His “blooded” animals (Enaima) roughly correspond to what we now call vertebrates, and his “bloodless” animals (Anaima) to invertebrates. The distinction was not perfect by modern standards, since some invertebrates do have oxygen-carrying fluids, but it captured something real: the presence of a red, iron-rich fluid running through visible vessels was a reliable marker for a whole cluster of anatomical features, including an internal skeleton, a heart, and a liver.
Among the blooded animals, Aristotle recognized several broad groups. Viviparous quadrupeds, the animals we would call mammals, gave birth to live young and had four legs. Birds formed a distinct group defined by feathers, beaks, and egg-laying. Fish lived in water and breathed through gills. Cetaceans, meaning whales and dolphins, gave him some trouble: they lived in water like fish but breathed air and bore live young. Aristotle correctly placed them closer to viviparous quadrupeds than to fish, an insight that took European natural history another two thousand years to fully accept. Snakes, lizards, and similar creatures formed another cluster, roughly equivalent to modern reptiles and amphibians, though Aristotle did not draw a sharp boundary between the two.
The bloodless animals were divided into four groups as well. Soft-bodied creatures (Malachia) included what we call cephalopods: octopuses, squid, and cuttlefish. Soft-shelled creatures (Malacostraca) were crustaceans like crabs, lobsters, and shrimp. Hard-shelled creatures (Ostracoderma) covered animals with external shells, including snails, clams, and sea urchins. And jointed creatures (Entoma) were the insects and arachnids. Aristotle was aware that these groups were not all equally tidy, and he sometimes noted intermediate forms that blurred the lines.
How He Actually Sorted Them
Aristotle did not pick a single trait and sort every animal by it. That method, called division by dichotomy, had been championed by his teacher Plato, who liked to split categories in half again and again until you arrived at a single species. Aristotle explicitly rejected this approach in his Parts of Animals, arguing that it forced organisms into artificial slots and ignored the fact that real groups of animals share clusters of features, not just one. A bird is not simply “a flying thing with feathers.” It also lays hard-shelled eggs, has a beak, has a certain arrangement of internal organs, and moves in a particular way. Focusing on just one trait would scatter naturally similar animals across unrelated categories.
Instead, Aristotle used what he called “the more and the less,” a method based on degrees of variation within a shared body plan. Among birds, for instance, some have long legs and some have short legs, some have broad wings and some have narrow wings, but they all share the basic features that make a bird a bird. He grouped animals first by these broad shared body plans and then distinguished them within a group by finer differences in the size, shape, and proportion of their parts. This is surprisingly close to the logic behind modern comparative anatomy, where organisms are grouped by homologous structures rather than by superficial similarities.
Reproduction was another major sorting criterion. Aristotle paid close attention to how organisms produced offspring: through live birth, through eggs laid on land, through eggs laid in water, through budding, or through what he believed was spontaneous generation from decaying matter. He described reproductive anatomy, mating behaviors, and gestation periods in considerable detail, and he used differences in reproductive strategy as evidence for how closely or distantly related two groups of animals were.
Plants and the Vegetative Soul
Aristotle said less about plants than about animals, though he clearly considered them a major division of living things. In his framework, plants possessed only the vegetative soul, the capacity for nutrition and reproduction, and lacked sensation and voluntary movement entirely. This made them the simplest form of life and the foundation upon which all other living functions were built.1Aristotle on Teleology. Teleology and Organisms i: General Principles
The concept of the vegetative soul became enormously influential long after Aristotle’s death. In Aristotelian traditions, it served as a principle for understanding the basic activities of all life, and discussions of it shaped how European scholars understood plants and living bodies well into the early modern period.2Max Planck Institute for the History of Science. Vegetation and vegetative powers His student Theophrastus picked up the botanical work where Aristotle left off, producing detailed surveys of plant anatomy, ecology, and uses that are sometimes called the founding texts of botany. Theophrastus sorted plants into trees, shrubs, under-shrubs, and herbs, a simple scheme based mostly on growth habit and woodiness that persisted in European natural history for centuries.
The Ladder of Nature Myth
One of the most common things people “know” about Aristotle’s classification is that he arranged all organisms on a scala naturae, a single ladder running from the lowest forms of life up through plants, animals, and finally humans at the top. This image of a Great Chain of Being became hugely important in medieval and Renaissance thought, and it is often attributed directly to Aristotle. The reality is more complicated.
Aristotle did describe a sequence of soul functions, from the vegetative to the sensitive to the rational, and he clearly thought humans had capacities that other animals lacked. But modern scholarship on this point suggests the sequence was not ordered by honor or value. Instead, it was ordered by distribution: the vegetative functions are found in the widest range of organisms, sensitive functions in a narrower range, and rational functions in the narrowest. Greater complexity, in Aristotle’s view, was not a mark of superiority. He argued that simplicity was actually closer to an ideal: psychological complexity was, for him, a sign of creatures that fall short of perfection, much as he discussed the complex motions of planets as evidence of their imperfection compared to the simple circular motion of the outermost sphere.3Dumb Beasts and Dead Philosophers. On the Disadvantages of Being a Complex Organism: Aristotle and the scala naturae
This matters because it changes the whole picture of what Aristotle was doing. If he was not arranging life on a value hierarchy, then his classification was more genuinely biological and less anthropocentric than the medieval thinkers who later borrowed his framework made it seem. Animals, in Aristotle’s account, were not “lesser” beings striving upward toward human perfection. They were well-adapted to their own modes of life, and their lack of what Aristotle called eudaimonia, the kind of flourishing tied to human reason and virtue, did not mean they were lacking in their own form of success.3Dumb Beasts and Dead Philosophers. On the Disadvantages of Being a Complex Organism: Aristotle and the scala naturae An octopus doing what octopuses do is not failing at being a person. The hierarchical Great Chain reading was largely imposed on Aristotle by later interpreters, especially Neoplatonists and Christian theologians who needed a ranked cosmos with God at the top.
Observation, Dissection, and Getting Things Wrong
What made Aristotle’s classification unusual for its time was the sheer amount of firsthand observation behind it. His Historia Animalium runs to ten books and describes the anatomy, behavior, diet, and habitats of hundreds of species. He dissected animals regularly, including marine creatures collected from the coast near Lesbos, where he spent time between his years at Plato’s Academy and his later tutoring of Alexander the Great. He described the chambered stomach of ruminants, the reproductive anatomy of sharks, and the social behavior of bees with a level of detail that was not matched in European writing for nearly two millennia.
He also got things wrong, sometimes spectacularly. He believed that some organisms, particularly insects and certain marine creatures, arose through spontaneous generation from mud, dew, or decaying matter. He thought the function of the brain was to cool the blood, assigning the seat of intelligence to the heart instead. His classification of certain borderline organisms, like sponges and sea anemones, wavered because their features genuinely did not fit neatly into his animal-versus-plant framework, though he perceptively noted that some of these creatures seemed to occupy a middle ground. These errors were not signs of carelessness; they reflected the limits of what naked-eye observation and the available theoretical tools could do in the fourth century BCE.
His method of grouping organisms by clusters of shared features, rather than by a single defining trait, was the most forward-looking aspect of his approach. When Linnaeus built modern taxonomy in the eighteenth century, he imposed a strict hierarchy of kingdom, class, order, genus, and species that Aristotle never used in quite that formal a way. But the underlying principle, that you sort organisms by looking at which ones share the most structural similarities, is there in Aristotle’s work. He was reaching for what we would now call a natural classification, one that reflects real relationships among organisms rather than arbitrary human choices about which trait to privilege.
Why Form Follows Function in Aristotle’s Biology
Aristotle’s teleological view of nature was inseparable from his classification. He did not just note that fish have gills; he wanted to explain why fish have gills in terms of what the gills are for. For him, the elements that compose an organism’s body, from basic tissues up through complex organs, exist for the sake of the whole organism and its way of life.1Aristotle on Teleology. Teleology and Organisms i: General Principles A beak exists so a bird can feed; a shell exists so a clam can protect itself. The starting point for any biological explanation was identifying the organism’s functions: what does it eat, how does it reproduce, how does it move, how does it perceive its environment? Classification followed from those questions, because animals that shared the same set of functions tended to share the same body plan.
This functional approach gave Aristotle’s system a coherence that a purely morphological scheme would have lacked. It also meant he sometimes grouped organisms in ways that cut across superficial appearance. Dolphins and whales looked like fish on the outside but functioned, in reproduction and breathing, like land mammals. Bats had wings like birds but reproduced and nursed their young like quadrupeds. Aristotle noticed these mismatches and used function, not appearance, to decide where an organism belonged. The survival and reproduction of each species was, for him, the anchor of its explanation. This is not evolution by natural selection, obviously; Aristotle thought species were fixed and eternal. But his insistence that you classify an organism by what it does, not just by what it looks like, anticipated a principle that later biologists would rediscover.
Intermediate Forms and the Limits of Neat Categories
One of the more interesting aspects of Aristotle’s biological writings is his repeated acknowledgment that nature does not always cooperate with tidy classification. He described organisms that sat on the boundary between major groups: sponges, which were attached to rocks like plants but responded to touch like animals; sea anemones, which had mouths and seemed to feed actively but could not move; and certain worm-like marine creatures that defied easy placement. Rather than ignoring these cases or forcing them into one group, Aristotle tended to discuss them as evidence that nature was continuous, that there were no sharp jumps between the plant kingdom and the animal kingdom but rather a gradual transition.
He also recognized gradations within groups. Not all animals perceived the world in the same way. Some had all five senses, while others had only touch. Some could move from place to place, while others, like certain shellfish, spent their entire lives anchored to a surface. He used these gradations to establish subgroupings and to discuss which organisms were more or less complex in their capacities, though, as noted above, “more complex” did not mean “better” in his framework.
This willingness to sit with borderline cases rather than flatten them into a system is something that distinguishes Aristotle from many of his successors. Medieval scholars who inherited his framework tended to prefer sharp boundaries and hierarchical ranks, partly because a tidy ladder of nature fit well with theological ideas about divine order. Aristotle himself seems to have been more comfortable with biological messiness. He was trying to describe what he actually saw, even when what he saw refused to fit his categories cleanly, and that empirical honesty is one of the reasons his biological works held up better than his physics over the long run.
What Aristotle Did Not Do
It is worth being clear about what Aristotle’s classification was not. He did not use binomial nomenclature, the two-part Latin naming system that Linnaeus formalized. He did not have a single fixed hierarchy of ranks like kingdom, phylum, class, order. He did not think species changed over time. And he did not have access to microscopy, genetics, or any of the tools that underpin modern systematics. His groupings were based entirely on what could be seen with the naked eye, supplemented by reports from fishermen, travelers, and beekeepers whose reliability he sometimes questioned.
He also did not produce a single, clean chart or diagram of his classification. His biological ideas are scattered across multiple works written over years, and scholars have spent centuries trying to reconstruct a unified system from texts that were probably never meant to be read as one consistent document. Some of the groups he described are remarkably close to modern taxa. His Malacostraca, for instance, maps well onto the crustacean subphylum. Others are grab-bags that no modern biologist would accept. The point is not that Aristotle got the right answer by modern standards; it is that he asked the right questions, looked at real organisms with real attention, and built a framework that remained the starting point for Western biology until the scientific revolution.