You’ve probably heard a version of it before. It’s the childhood chant, the nursery rhyme, the sentence that feels like it could go on forever:

“This is the cat that chased the rat that ate the cheese that was stored in the barn that Jack built.”

Each time you add a new “that”, you add another layer to the story, like adding a new car to a toy train. You could, in theory, keep going for hours, days, or even years, and the sentence would still be grammatically correct. This fascinating, seemingly magical property of language isn’t just a quirky feature; it’s a fundamental concept in linguistics called recursion, and some of the world’s most influential linguists believe it’s the secret ingredient that makes human language truly special.

What Exactly is Linguistic Recursion?

At its core, recursion is the process of embedding a structure inside another structure of the same type. Think of it like a set of Russian nesting dolls (Matryoshka dolls). You open one doll to find a slightly smaller doll, and inside that one is an even smaller doll, and so on. In language, we don’t nest dolls; we nest phrases and clauses.

Let’s break down our classic example to see how it works:

We start with a simple main clause:

• This is the cat.

Now, we want to add more information about the cat. We can do this by embedding a relative clause—a phrase that acts like an adjective—into our sentence.

• The cat chased the rat. → that chased the rat

We slot this new clause right after the noun it describes (“the cat”):

• This is the cat [that chased the rat].

The beauty of recursion is that we can do it again. We can now describe “the rat” by embedding another relative clause right after it:

• The rat ate the cheese. → that ate the cheese

Slotting this in gives us:

• This is the cat [that chased the rat [that ate the cheese]].

And so on, infinitely. Each new relative clause is a doll nested inside the previous one. This ability to repeatedly apply the same simple rule to create ever more complex structures is the engine of linguistic creativity.

It’s Not Just a Party Trick

While the “cat that ate the rat” example is a bit exaggerated, we use recursion constantly in our everyday speech without even noticing. It’s what allows us to express complex relationships and add detail and nuance to our thoughts.

Consider these common examples:

• Possessives: “I found my mom’s friend’s car keys.” Here, a possessive phrase (“friend’s”) is nested inside another possessive phrase (“mom’s”).
• Prepositional Phrases: “She left the book on the table in the corner by the window.” A prepositional phrase (“by the window”) modifies a noun within another prepositional phrase (“in the corner”), which modifies a noun in yet another (“on the table”).
• Reported Speech: “He said that she thinks that I’m wrong.” We have a clause (“that I’m wrong”) embedded within another clause (“that she thinks…”), which is itself embedded in the main sentence. This is how we talk about what other people think, know, or believe.

Without recursion, our language would be stuck in the realm of simple, disconnected statements: “This is a cat. The cat chased a rat. The rat ate cheese.” It’s recursion that weaves these simple threads into a rich, complex tapestry of meaning.

The Cornerstone of Human Language?

This is where things get really interesting. For decades, linguists like Noam Chomsky have argued that recursion is not just another tool in the linguistic toolkit—it might be the defining feature of human language.

Their argument is based on a powerful idea: infinity from a finite set. We have a finite number of words in our vocabulary and a finite set of grammatical rules. Yet, with these finite tools, we can produce and understand a potentially infinite number of unique sentences. How is this possible? Recursion is the key.

This generative power seems to be uniquely human. Animals have incredible communication systems of their own. For example:

• Vervet monkeys have distinct alarm calls for different predators (“leopard!” “eagle!” “snake!”).
• Bees perform a complex “waggle dance” to communicate the precise direction and distance of a food source.
• Whales sing complex songs that evolve over time.

However, these systems are generally considered “finite.” A monkey has a set number of calls for a set number of meanings. It cannot combine its “leopard” call and its “snake” call to create a new warning like, “Watch out for the leopard that lives near the snake’s den.” A bee cannot dance about the flower that was visited yesterday by another bee. They lack the grammatical machinery to embed one concept inside another.

Recursion, therefore, might be the cognitive leap that allowed our ancestors to move beyond simple alerts and statements to complex narratives, hypothetical scenarios, and abstract thought.

The Brain’s Breaking Point

If language is grammatically infinite, why can’t we actually utter a sentence that lasts forever? And why does a sentence like this give you a headache?

“The cheese that the rat that the cat that the dog chased bit ate was moldy.”

The answer lies in the distinction between linguistic competence and linguistic performance.

Competence is our abstract, unconscious knowledge of the rules of grammar. In this abstract sense, the sentence above is perfectly grammatical. The rules of recursion have been followed correctly.

Performance, however, is the actual use of language in the real world. Our performance is limited by practical constraints like breath, time, and, most importantly, working memory. Our brains are like computers with a limited amount of RAM. When we have too many nested clauses, especially “center-embedded” ones like in the cheese example, our short-term memory overflows. We lose track of which verb belongs to which subject, and the sentence collapses into a confusing mess.

So while our grammar allows for infinite sentences, our brains gently apply the brakes, keeping our communication manageable and efficient.

The Infinite Power in a Simple Rule

The next time you hear yourself saying, “the guy from the office I was telling you about”, take a moment to appreciate the incredible cognitive gymnastics you’re performing. You’re using recursion. You’re nesting ideas inside of ideas, effortlessly weaving together complex thoughts that would be impossible to express with simple, linear statements.

The sentence that never ends isn’t just a child’s game. It’s a window into the very structure of the human mind and the simple, elegant rule that unlocks the infinite, creative potential of language.