Thomas Kuhn’s The Structure of Scientific Revolutions, published in 1962, was itself a revolutionary approach to the philosophy of science.

The book was both influential and controversial.

Born in 1922, Kuhn began his career as a physicist before turning to the history of science.

He was interested in how scientists approach their daily work, and in thinking about the question of how science develops over time.

Kuhn saw sciences progressing in two alternating phases: one he called normal and the other he called extraordinary (or revolutionary).

Scientific development is traditionally thought of as simply moving faster when a discovery is made, like the discovery of bacteria, or the realisation that the earth revolves around the sun, not the other way around.

But for Kuhn, the normal and extraordinary phases of science aren’t just different speeds of discovery, but fundamentally different approach to scientific work.

Let’s look at the normal phase first.

The traditional view of science looks something like this.

There are textbooks describing observations, laws and theories that are taught to students.

Science within this body of knowledge is simply a ‘constellation of facts, theories, and methods’ contained in those textbooks and the way they are taught.

Great scientists are those that have contributed to that body of knowledge.

This stockpile of scientific knowledge grows over time.

Science then is cumulative.

Discoveries are added to the pile – sometimes at quicker speeds than others.

Normal science – the day to day of scientific activity – is based on this stockpile, this body of knowledge.

Kuhn calls this a paradigm.

The paradigm has approaches, laws, theories, ways of applying them, instruments – a network of assumptions and facts that work together, a model, a pattern.

The paradigm ‘provides a map whose details are elucidated by mature scientific research. And since nature is too complex and varied to be explored at random, that map is as essential as observation and experiment to science’s continuing development. Through the theories they embody, paradigms prove to be constitutive of the research activity’.

Kuhn sometimes calls this a disciplinary matrix.

The paradigm contains a particular selection of facts like positions in astronomy, the mass of materials, wave lengths, boiling points, acidity levels, that have ‘shown to be particularly revealing of the nature of things’.

There are a class of instruments, machines, that can measure these things and explore them further, plus a class of engineers that seek to improve these machines based on acquiring more sophisticated readings.

There is a class of instruments that both describe these and use the principles to create more sophisticated instruments.

Finally, there are problems, gaps in knowledge, areas where theories need expanding or confirming.

Kuhn refers to this ‘normal science’ simply as ‘puzzle solving’.

The puzzles are assumed to have a solution based on this collection of theories, facts, scientists and instruments.

There are assumptions that direct the puzzle solving – most importantly the assumption that this selection of pieces will solve the puzzle.

But sometimes they don’t.

Sometimes the pieces don’t fit.

And sometimes the rules of the puzzle need to be abandoned.

Kuhn writes, ‘Throughout the eighteenth century those scientists who tried to derive the observed motion of the moon from Newton’s laws of motion and gravitation consistently failed to do so. As a result, some of them suggested replacing the inverse square law with a law that deviated from it at small distances. To do that, however, would have been to change the paradigm, to define a new puzzle, and not to solve the old one. In the event, scientists preserved the rules until, in 1750, one of them discovered how they could successfully be applied. Only a change in the rules of the game could have provided an alternative’.

An anomaly occurs when a piece of the puzzle just won’t fit. This can lead to a crisis in the paradigm and a search for a new one. Increasingly problematic anomalies – and a consensus that there is an anomaly – leads to increased debate, different perspectives on what could change, disagreement and a new type of experimenting.

Kuhn writes that, ‘When, for these reasons or others like them, an anomaly comes to seem more than just another puzzle of normal science, the transition to crisis and to extraordinary science has begun. The anomaly itself now comes to be more generally recognized as such by the profession. More and more attention is devoted to it by more and more of the field’s most eminent men’.

This leads to the pursuit of extraordinary science, which might lead to a revolution.

The extraordinary phase consists of a ‘loosening’ of the rules of research, more experimentation and creativity, more randomness, trying new and unique experiments, and the deconstruction of stereotypes, the reading of different philosophies.

Kuhn writes that, ‘The proliferation of competing articulations, the willingness to try anything, the expression of explicit discontent, the recourse to philosophy and to debate over fundamentals, all these are symptoms of a transition from normal to extraordinary research’.

This, if successful, leads to a complete change of perspective, a paradigm shift.

Where a completely new perspective – like Newton’s laws of physics, or Einstein’s relativity, or quantum mechanics – can appear.

Kuhn sometimes describes the state of affairs that leads to this as pre-paradigmatic because it’s like the Wild West, there’s a lack of consensus, the trying of anything, a storm of experimentation, and competition between different schools of thought.

A shift in paradigm is like the shift in perception from the rabbit to the duck in a Gestalt image. Nothing has changed apart from your psychological perspective – two people with the same sensory impressions can see different things.

Ptolemy saw the sun revolving around the earth. Nothing changed when Copernicus described it as being the other way around – it was just a change in perspective.

In a paradigm, scientists look in a particularised way with a limited constellation of assumptions and ideas at, for example, the equipment in the lab, the materials around them, at their colleagues and the conversations they have with them.

Let’s take an example.

The discovery of oxygen in the 18th century was a revolution in chemistry.

Before this, the dominant view – the paradigm – inherited from the Greeks, was that there were four distinct elements – earth, water, fire, and air.

The leading theory that arose from this was that combustible materials like wood contained something called phlogiston that enabled them to burn.

When they burned they released their phlogiston.

But when ore is heated and turned to metal it gains weight. For the most part, this anomaly was ignored, or explained as phlogiston from the coal transferring into the metal.

When metal rusted you could see the phlogiston leaving it.

But this led to another problem. Rusted metal weighs more than normal metal.

Again, this was largely ignored because it couldn’t be solved with the pieces of the puzzle supplied by the reigning phlogiston paradigm.

Two things combined to create a crisis – the increase in the precision and importance of using weights, and the invention of pneumatic chemistry that used equipment like air pumps to compress air and see the effects of gases.

Air pumps meant you could burn something and see air being released.

This meant the phlogiston theory was put under increasing strain. There was an increasing amount it couldn’t answer.

Two chemists – Joseph Priestley and Antoine Lavoisier – conducted innovative experiments during this time, focusing on burning different materials and weighing the results.

Priestly discovered that heated mercury calx produces air when burned, and that if you burn a candle in this air it burns even brighter than in normal air.

What was special about mercury calx is that it burned without coal, without fuel, with just a flame or the light from a magnifying glass.

Yet it produced something, without phlogiston.

Lavoisier saw that while some metals increased their weight when burned (because of what we now know as oxygenation) others lost weight.

He realised it wasn’t phlogiston, but the air entering into and leaving the metal.

He named this oxygen.

Lavoisier discovered in 1777 the oxygen theory of combustion – a paradigm shift that changed the rules of chemistry.

Now, rather than chemists focusing on the phlogiston theory, they focused on the oxygen theory and the possibility that rather than four elements, the world was made up of many more.

We can see something similar going on in the Copernican revolution in the 16th century.

Ptolemy’s system – that the earth was at the centre of the universe – was pretty good. It predicted the position of stars and can actually still be used today.

But it often failed. It couldn’t quite predict the position of planets or equinoxes.

Over the centuries star gazers knew that something was wrong.

When Copernicus realised that the earth went around the sun it started a paradigm shift that altered a huge range of scientific assumptions.

Newton did the same for gravity.

Aristotle thought that objects moved towards the ground because it was in their nature. Newton saw that all bodies attracted others, discovering the general laws of physics.

Kuhn notes how paradigm shifts are usually sparked by young, creative and eccentric individuals, or scientists new to the field, people not embedded so deeply in the assumed paradigm.

They see things in completely new ways.

Paradigms shift how we perceive the world – what we see when we look at the stars or a burning piece of wood. And every historical society assumes their worldview is the correct one. But if history tells us anything, it’s that those assumptions are often wrong.