A couple of years ago, I was preparing a small group of students for a debate on whether the ban on smoking in public places should be repealed. We started out with a general discussion about cigarettes. “Teachers are always telling us not to smoke,” one student said. “But I’ve seen some of them smoking in their cars during break time. Why should we listen to them?”

“That’s true,” said another. “But they’re right to tell us not to smoke. Smoking is bad for you. Our teachers know it’s bad and probably don’t have the willpower to stop themselves.”

The rest of the class nodded wisely at the truth of this argument. I was so impressed that I temporarily forgot about intervening to defend my colleagues in the face of charges of hypocrisy and weakness. This student had demonstrated a skill that has always been extremely valuable, and which is arguably even more vital in a world where the amount of information we are presented with is increasing all the time, but where the amount of truth remains constant. She had shown that she could think critically.

Defining critical thinking is not easy; indeed, definitions vary hugely. Some say the top three levels of Bloom’s Taxonomy (analysis, synthesis and evaluation) make up the essence of critical thinking. Academic Robert Ennis, who has written extensively about critical thinking, is more specific: he defines it as “reasonable reflective thinking, focused on deciding what to believe or do”. Cognitive scientist Daniel Willingham sees it as reasoning, decision-making and problem-solving that is effective, novel and self-directed. The thought processes that my student used to evaluate her classmate’s answer offer a practical example of critical thinking.

Her approach was novel and self-directed - there was no input from me. She analysed and evaluated an alternative point of view and made an effective decision about its value. Although she didn’t know it, she had identified and corrected a relatively complicated philosophical theory called the tu quoque logical fallacy. Not bad, eh?

We can all think of ways that we use (or don’t use) critical thinking in our lives, and of the many ways in which such thought processes are valuable. Given the benefits of such skills, it’s understandable that attempts have been made around the globe to design curricula and teaching programmes to instil them. England’s former education secretary David Blunkett was a believer, and when the new national curriculum was unveiled in 2000, it included five thinking skills.

What do you know?

A flaw with many of these critical thinking programmes, however, is that they fundamentally misunderstand the nature of cognitive skills. It is assumed that critical thinking is a generic technique that, once learned, can be applied to any area or body of knowledge. Edward de Bono’s well-regarded 1974 Cort programme, for example, suggests that critical thinking is a skill akin to riding a bicycle.

Yet the work of scientists such as Daniel Willingham suggests that thinking is considerably more complex. The processes of thought are intertwined with the content.

We may be good at thinking critically in one domain. But what the research shows is that we can’t transfer this skill to a domain where we have little knowledge. Most people recognise that it is possible to be a great critical thinker in science, for example, but a poor one in languages. A PhD history researcher will be able to think critically about a historical topic, but they won’t necessarily be so good at thinking critically about how to fix their faulty electrical wiring, or how to buy a reliable second-hand car.

Consider critical thinking in designing an experiment: it is crucial to know which variables you have to control for. But knowing this inevitably depends on domain knowledge. Do you need to control for gender? For age? For weight? For height? Only knowledge of the domain can give you the correct answer.

The vital role of knowledge in cognition has been understood for decades now. A seminal study into the nature of skill was carried out by a Dutch researcher, Adriaan de Groot. He wanted to know what separated a chess grandmaster from an average club player. His research found that the main difference was not the possession of superior abstract reasoning skills, nor the possession of certain maxims about the game. The main difference was knowledge.

Grandmasters had thousands of chess positions committed to memory. When they looked at a chess board, they didn’t reason abstractly through all the best possible moves. Instead, they used their memory to identify similar positions they had encountered in the past, and the best moves to use in those situations. What looked like reasoning was, in fact, recall.

Since de Groot’s work, many other researchers have replicated this finding and analogous ones in other domains. There is now a vast body of research into how humans acquire skill and expertise which confirms the importance of domain knowledge - and the relative weakness of generic “how to” strategies.

The long and the short of it

Domain knowledge is so important because of the way our memories work. When we think, we use both working memory and long-term memory. Working memory is the space where we take in new information from our environment; everything we are consciously thinking about is held there. Long-term memory is the store of knowledge that we can call up into working memory when we need it. Working memory is limited, whereas long-term memory is vast. Sometimes we look as if we are using working memory to reason, when actually we are using long-term memory to recall. Even incredibly complex tasks that seem as if they must involve working memory can depend largely on long-term memory.

When we are using working memory to progress through a new problem, the knowledge stored in long-term memory will make that process far more efficient and successful. In the case of the chess grandmasters, for example, there is no way that any human could rely on working memory alone to sort through all the possible options and permutations, because there are billions of ways a chess game can play out. Instead, grandmasters use knowledge of chess games that they have stored in long-term memory to narrow down the possible options for a move. They can then solve this more manageable task in working memory.

Something similar is true of driving. The safest people behind the wheel have automated the processes so that they have all their working memory free to deal with novel information, such as a child running out into the street. The same holds true for any complex reasoning task. The more parts of the problem that we can automate and store in long-term memory, the more space we will have available in working memory to deal with the new parts of the problem.

If all we had to rely on for thinking was working memory, we wouldn’t, as a species, be capable of all that we are. Thus, a major part of getting students to think critically doesn’t actually involve them using working memory to reason at all. It involves them committing important knowledge to long-term memory so that they are able to free up space in working memory when they encounter new problems.

Swedish scientist K Anders Ericsson carried out most of the research that social sciences journalist and author Malcolm Gladwell discussed in his book Outliers. He showed that experts in many different domains require years of practice to achieve such expertise (although he has taken exception to Gladwell’s 10,000 hours rule). He highlights the theoretical problem with many generic thinking courses: “Modern educators have trained many generalisable abilities such as creativity, general problem-solving methods, and critical thinking,” he writes. “However, decades worth of laboratory studies and theoretical analyses of the structure of human cognition have raised doubts about the possibility of training general skills and processes directly, independent of specific knowledge and tasks.”

For example, research on thinking and problem solving shows that successful performance depends on special knowledge and acquired skills; studies of learning and skill-acquisition show that improvements in performance are primarily limited to activities in the specific domain.

Indeed, one of the major problems with many of these programmes is that although they claim to improve critical thinking in general, too often their improvements are limited to the specific tasks and areas on the programme. In order to improve critical thinking skills across domains, you have to teach knowledge from across domains - which obviously can’t be done in one particular course or programme. To the extent that teaching critical thinking is possible, it might not look that different from traditional subject teaching.

Face up to the facts

In practice, I encountered this problem a lot when I coached my school’s debating team. When we explored motions relating to the smoking ban or the length of school holidays, my students were capable of coming up with the kinds of excellent insights I outlined at the start of this article. If we had a debate on whether school uniforms should be abolished or not, my students would reliably employ all of the important critical thinking tools. With little or even no prompting, they would generate lots of ideas about the topic (school uniform promotes belonging, it’s smart, it reduces individuality, it makes people feel uncomfortable) and look at the issues from multiple perspectives (we might prefer not to have a uniform, but our teachers disagree because it discourages bullying). They were also able to bring a range of evidence to bear, such as the cost of school blazers, comparisons between different schools and citations from the uniform policy. In short, they employed some sophisticated critical thinking tools.

But when we moved on to a topic outside their comfort zone, one they were less knowledgeable about - for example, whether the monarchy should be abolished, or whether capital punishment should be reintroduced - the very same students would clam up. They would no longer spontaneously consider multiple perspectives, support their arguments with evidence or identify logical fallacies; even when prompted, they were unable to think in the same sophisticated ways they had just demonstrated with the previous topic.

My students simply didn’t have the domain knowledge to be able to think critically about these issues. The problem wasn’t that they had forgotten to apply maxims that they had been taught, or that they were too lazy to bother, or that they were having an off day. Likewise, all the prompting and guidance in the world from me wasn’t going to help them. And getting more practice with topics they did understand wasn’t going to be very useful either. What these students really needed was more domain knowledge about these particular topics.

Paradoxically, it turns out that a major way that students get better at critical thinking is by committing facts to long-term memory. The paradox here may well be a result of the confusion of aims and methods. Getting all students to be critical thinkers is a desirable aim. But “getting all students to be critical thinkers” may not be the best method of achieving it. We need to remember that there is a difference between practising a discipline and learning a discipline. Sometimes, learning a discipline doesn’t look very much like practising it at all. Asking students to practise critical thinking might not help them to get better at it, whereas helping them to master domain knowledge will. Knowledge is the fuel on which critical thinking runs - asking students to practise this skill without knowledge is like asking them to drive a car with no petrol.

Although critical thinking may be impossible to teach directly or quickly, it is an achievable goal. And given its value, it is undoubtedly one of the most important aims we can have as educators.

Daisy Christodoulou is an educationalist and research and development director at UK academy chain Ark. Her book Seven Myths About Education is published by Routledge, priced pound;14.99

Let’s get critical

Guide students towards a deeper understanding of your subject with these critical thinking resources, all available from TES Connect’s extensive online resources archive.

Introduce primary classes to philosophical teasers with these questions based on Ian Gilbert’s The Little Book of Thunks. bit.lytesthunks

Get to the heart of modern foreign languages by asking students to consider communication using critical thinking. bit.lywhatislanguage

Use these role cards to differentiate tasks and develop thinking skills for any subject, debate or discussion. bit.lyrolecards

Check learning through questioning with this Bloom’s Taxonomy wheel. bit.lybloomswheel

Draw ideas and inspiration from this insightful video on using critical thinking in the classroom. bit.lycriticalvideo

Try a video guide to providing students with the critical thinking skills required to succeed in school. bit.lythinkingforschool

Plenty more critical thinking resources can be found on TES Connect. Explore at bit.lytescriticalthinking