Thought might not be dependent on language, according to new research published in the Proceedings of the National Academy of Sciences.
Cleverness and language might not be as closely connected as once thought
A UK team has shown that patients who have lost the ability to understand grammar can still complete hard sums.
This suggests mathematical reasoning can exist without language.
The study undermines the assumption that language is the key quality that makes our thought processes more advanced than those of other animals.
"We are kicking against the claim that it is language that allows you to do other high order intellectual functions," lead research Rosemary Varley, from the University of Sheffield, told the BBC News website.
The researchers made the discovery by studying three patients who were suffering from severe aphasia - they had lost the ability to understand, or produce, grammatically correct language.
For example, although they understood the words "lion", "hunted" and "man", they could not tell the difference between the sentences "The lion hunted the man" and "The man hunted the lion".
But when they were presented with sums like 52 minus 11 and 11 minus 52, which were structured in a similar way, they had no problem.
"Our patients can clearly do those problems which show the same reversibility," said Dr Varley. "So that shows they have a good insight into these very abstract principles.
"Despite profound language deficits these guys showed advanced cognitive abilities, which indicates considerable autonomy between language and thinking."
The new findings contradict previous studies which used brain imaging techniques to work out how people process mathematics.
A French-led team found that calculations lit up the left frontal lobe, an area of the brain known to make associations with words. But Dr Varley is not convinced by this research.
"The problem with functional brain imaging is you don't really know what your subjects are doing when they are in the scanner," she said. "If you give them a sum they might be reading the numbers aloud in their head.
"But that is not to say that language is necessarily a part of mathematics."
If Dr Varley is correct, it again raises the question of what makes humans different.
According to many academics, people are much cleverer than other animals because language gives them a higher order of thought. But these findings suggest cleverness and language might not be as closely connected as once assumed.
Elizabeth Brannon, of Columbia University, US, wrote in a commentary article: "A promising avenue for further exploring this hypothesis is to look for precursors of social reasoning and mathematical syntax in nonhuman animals."