Scientists have discovered whether humans need language for logical thinking

Two people with severe speech impairments following a stroke successfully identified hidden patterns in numbers and geometric shapes. Meanwhile, scans of healthy participants showed that the language areas of the brain did not become more active during logical reasoning.

This does not mean that language plays no part in thinking at all. The study focuses on specific tasks involving formal logic. However, it confirms an important point: a person who finds it difficult to speak or understand speech has not necessarily lost the ability to reason.

How scientists separated thinking from language

It is difficult to test whether a person can think without words. Most logic tests require participants first to read the question and then to explain their answer. In such cases, it is unclear exactly what the experiment is testing: the ability to reason or language proficiency.

Scientists from the Massachusetts Institute of Technology and University College London developed tasks that required almost no speech.

Two people who had suffered a stroke took part in the clinical part of the study. The damage affected the areas of the brain responsible for language processing. As a result, the participants found it very difficult to understand and pronounce words.

They were presented with logic puzzles that could be solved without reading lengthy instructions or verbal explanations.

What tasks did the participants complete?

In one experiment, participants were shown two rows of numbers. They had to work out the hidden rule that transformed the first row into the second.

For example, the rule might have been to arrange the numbers in reverse order or to remove all numbers above a certain value. Afterwards, the participants had to apply the rule they had identified to a new example.

In another task, they were shown a grid of geometric shapes. They had to determine which element completed the pattern.

The tasks gradually became more complex. Despite severe speech impairments, both participants solved them with roughly the same level of success as those in the control group.

Moreover, they were able to demonstrate the rule they had discovered using drawings or gestures. In other words, the problem lay in verbal explanation, not in the reasoning itself.

What the brain scans revealed

Separately, the researchers carried out functional MRI scans on healthy adults whilst they were solving logic puzzles.

First, the scientists identified the areas of the brain responsible for language processing in each participant. They then examined whether these areas showed greater activation when a person needed to draw a logical conclusion.

The participants solved two types of problems.

In inductive reasoning, they had to independently discover a hidden rule based on examples.

In deductive reasoning, they tested conclusions drawn from already known conditions. For example: ‘If the ball is red, then it is large. The ball is red. Does this mean it is large?’

The brain’s language network showed no noticeable increase in activity, either when searching for hidden rules or when verifying logical conclusions. This suggests that formal logic relies on other brain systems.

Does this mean that we do not think in words?

No. The study does not prove that language is completely unnecessary for thinking.

People can indeed use inner speech to retain information, make plans, control behaviour or break down a complex problem into steps. Words are also essential when we need to convey a thought to another person.

But internal speech, according to the new findings, is not the only way to reason. The brain can represent relationships between objects and rules in other forms — for example, as abstract diagrams, visual images or non-verbal structures.

The study did not establish exactly what this internal ‘code’ of thought looks like.

What is aphasia?

Aphasia is an impairment of the ability to understand or use language, which usually occurs following a stroke, head injury or other brain damage.

A person may experience difficulties with word-finding, sentence construction, reading or understanding spoken language. However, memory, attention, knowledge and the ability to reason may remain intact.

Severe aphasia is sometimes mistakenly perceived as a loss of intelligence. A person may respond slowly, use the wrong words or be unable to explain their thoughts at all – and those around them conclude that they do not understand anything.

New research shows why this conclusion may be incorrect. The ability to express a solution in words and the ability to arrive at the solution itself are not one and the same.

Why logic and language seemed inseparable

Language and logic do indeed share common features. Both a sentence and a complex line of reasoning can be broken down into smaller elements and then combined according to certain rules.

For example, several words form a sentence, whilst several simple propositions form a logical conclusion. Because of this, philosophers, linguists and neuroscientists have long debated whether language is the foundation of human thought.

However, there are also differences between them. Speech unfolds sequentially — one word after another. A logical problem may require the simultaneous comparison of several relationships, options and conditions.

The findings support the view that language helps to convey reasoning, but does not necessarily generate it.

Which brain systems are responsible for logic

The researchers also studied the so-called ‘multiple demands network’. This comprises several brain regions that are activated during complex tasks requiring attention, control and problem-solving.

This network was involved in inductive tasks, where a person had to independently identify a hidden pattern. However, it did not show the same level of activity during deductive reasoning.

The result was unexpected. It suggests that the brain probably does not have a single universal ‘centre of logic’. Different types of reasoning may rely on different systems.

What this research means for people with aphasia

The main practical significance of this work lies not in philosophy, but in our attitude towards people with speech impairments.

A person with aphasia can continue to play chess, solve puzzles, manage family finances, assess risks and make decisions. Yet those around them often judge their abilities solely on the basis of their speech.

The authors emphasise that language difficulties are not a reliable indicator of intelligence. This applies not only to people who have suffered a stroke, but also to those who stammer, have congenital speech characteristics or do not speak the language of those around them fluently enough.

The inability to formulate a response quickly does not necessarily mean a lack of thought.

What does artificial intelligence have to do with this?

The study also raises the question of the differences between the human brain and large language models.

Such systems are trained primarily on text and produce text-based responses. At the same time, they are capable of mimicking certain types of logical reasoning.

According to the new study, in humans, language and formal logic are more strictly separated. Comparing these two ways of processing information may help researchers better understand both human thinking and the limitations of artificial intelligence.

However, the study does not prove that language models actually think or, conversely, are incapable of reasoning. The AI was not directly tested in the study.

Limitations of the study

The main limitation of the clinical part is the participation of only two people with aphasia. Their results are indicative, but do not allow the conclusions to be automatically generalised to all patients with damage to language areas.

Furthermore, the researchers examined a limited set of formal tasks: rule-finding, numerical transformations, geometric matrices and ‘if–then’ logical constructs.

Everyday thinking is far more complex. When a person plans for the future, assesses the actions of others or makes an emotionally significant decision, they may simultaneously draw on language, memory, emotions and social experience.

Therefore, the precise conclusion is this: natural language is not a necessary condition for certain forms of abstract logical thinking.

Why this is important

The study distinguishes between two abilities that are often mistaken for one: the ability to reason and the ability to express that reasoning in words.

For science, this is a new argument in the centuries-old debate on the relationship between language and thought. For society, it is a reminder that speech difficulties do not make a person any less intelligent or capable.

The brain can use words as a convenient tool. But, according to the study, logical thought can still exist even when this tool is severely impaired.

Background

A team led by neuroscientist Evelina Fedorenko had previously shown that the brain’s language areas are not essential for certain other processes, including recognising categories of objects and reasoning about social situations.

This new study adds formal logic to that list. However, the authors do not claim to have discovered a separate ‘language of thought’. It remains unclear exactly how the brain encodes abstract rules without the involvement of ordinary speech.

Source

Hope Kean et al.,“Evidence from formal logical reasoning reveals that the language of thought is not natural language”, Proceedings of the National Academy of Sciences, 2026.