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, which he explored in The Structure of Scientific Revolutions.
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.
Normal science progresses under paradigms, but when anomalies appear, extraordinary science can lead to a paradigm shift that changes the fundamental underlying assumptions, norms and rules of scientific activity.
We can see this in the chemical revolution, when Joseph Priestly and Antoine Lavoisier weighed burning chemicals and gases and overturned the reigning phlogiston theory of combustion, replacing it with todays oxygen theory of combustion.