Science does not always progress smoothly — sometimes it works calmly within accepted rules, then a discovery suddenly overturns the whole way of thinking.

Science advances through periods of normal puzzle‑solving, followed by sudden paradigm shifts.

Kuhn used paradigm for the set of rules and assumptions accepted by a generation of scientists. Most of the time, researchers just solve small puzzles inside those rules. Occasionally, too many strange facts stop fitting, and the entire paradigm gets suddenly replaced with a completely new one.

When a paradigm shifts, not only theories change, but also what scientists consider an important problem. For example, before Copernicus, astronomers spent years explaining complicated planetary motions. After Earth was removed from the center, those same motions became simple effects of our orbit. Old questions lost their meaning, and new ones emerged. Kuhn shows that a scientific revolution redefines the very reality we study. The next step is to see how we recognize a crisis before it erupts.

Why it matters The term paradigm shift comes straight from this book and is used everywhere today.