In The Unique Animal, Rens Bod revisits an age-old philosophical question: what makes us human? His answer is that our uniqueness lies in unbounded recursion—which, according to Bod, is the defining feature that fundamentally distinguishes humans from all other animals.
Recursion is undoubtedly an elegant notion, with a long and rich intellectual history, which gained renewed momentum in the second half of the twentieth century through, among others, Douglas Hofstadter’s influential book Gödel, Escher, Bach, Mandelbrot’s theory of fractals, and Chomsky’s claim that recursion is the only truly distinctive property of the human language faculty.
Yet I wish to argue that this very search for uniqueness—for a single capacity that defines us—is a misleading enterprise. However intriguing recursion may be, it does not provide the solid foundation that some believe it does.
1. The problem of uniqueness
All animal species are unique. In that sense, we humans are unique as well—but not more so than other animals. Uniqueness is not rare; it is ubiquitous. The attempt to single out one exclusive feature in humans is therefore a peculiar, perhaps even pretentious, endeavor. The history of thought is full of such attempts—from Aristotle’s rational animal to Chomsky’s syntactic animal. But these efforts often reveal more about our desire to draw boundaries than about reality itself. We like to draw a sharp line between “human” and “animal,” while nature rarely complies.
2. Recursion and its limits
Recursion is without doubt a fascinating phenomenon, both mathematically and cognitively: the capacity to have thoughts about thoughts and to embed sentences within sentences. In theory, this allows infinite complexity to emerge from finite means—an influential idea.
But empirical reality is far less unbounded. Humans can process only a few levels of embedding—three, at most four—before losing track. In language, we lose the thread after the third subordinate clause; the same applies to reasoning and play. We can still follow that someone is pretending to pretend, but add yet another layer and we are lost.
Unbounded recursion therefore does not describe how the human brain actually functions. Rather, it is a theoretical idealization—a concept that helps describe grammars and other hierarchical patterns in behavior, without necessarily contributing to deeper understanding. (The trees of linguistics have more than once prevented us from seeing the forest of our cognitive capacities.)
3. The problem of testability
This leads to a more fundamental objection: unbounded recursion is not empirically testable. No experiment can demonstrate its existence, let alone falsify it, because every human performance is by definition finite. Nor can it be refuted, since any limitation can always be explained away as a matter of attention or memory. Thus the concept slips from the hands of science and drifts into the realm of metaphysics—into belief in something that may be true, but cannot be proven. For that reason, we must reject the thesis on rational grounds: not out of reluctance, but because a scientific explanation must be testable—or rather, falsifiable.
4. Beyond the linguistic lens
There is, moreover, another important factor at play: a widespread and persistent language bias in our thinking—a tendency I have pointed out before and written about elsewhere. Many researchers prefer to view cognitive phenomena through a linguistic lens. Because formal language systems are characterized by recursive structures, it is then often assumed that thought—and thus the human mind—must be recursive in nature. But cognition is more than language.
Music offers an interesting counterexample. Music also exhibits hierarchical structures—structures characterized by multiple levels of organization, repetition, variation, and symmetry. These properties are not uniquely human: songbirds and various marine mammals structure their vocalizations in ways that show striking similarities to human music and can be regarded as precursors of our musical capacity. Hierarchy, however, is not synonymous with recursion. Although the two concepts are closely related, there is an essential difference: recursion presupposes a hierarchical structure, but a hierarchy need not be recursive.
5. A different idea of uniqueness
Perhaps we should therefore abandon the search for a single, exclusive feature and understand uniqueness differently—not as something belonging solely to humans, but as an emergent phenomenon arising from the convergence of multiple capacities. What makes humans special, then, is not one isolated property such as recursion, but a specific combination of components that together form a unique whole. From this perspective, the question shifts from “What do humans have that other animals do not?” to “Which capacities make a species unique?” Our uniqueness is not a single essence, but an evolutionary pattern—a fabric of gradually developed capacities that together form the basis for culture, language, and music.
Epilogue — in relation to The Arrogant Ape
In The arrogant ape, Christine Webb dismantles the deeply rooted human tendency to overestimate our own exceptionalism. Humans, she argues, are not the pinnacle of evolution but one species among many—remarkable, yes, but not categorically superior. What Webb calls “arrogance” is precisely the urge critiqued in the column above: the desire to locate one decisive trait that elevates us above all other animals. In short, abandoning the myth of the uniquely gifted human does not diminish us. On the contrary: it situates us more accurately within the living world, as one expressive, musical, meaning-making species among many—remarkable not despite that continuity, but because of it.
Bod, R. (2025). Het unieke dier: Op zoek naar het specifiek menselijke. Amsterdam: Prometheus.
Webb, C. (2025). De arrogante aap: Waarom we niet zo uniek zijn als we denken. Amsterdam: Atlas Contact.
See also https://hdl.handle.net/11245.1/e38e36c0-4c95-4f7c-a3fa-5c890f5b5b7f.
