Kuhn introduces the concept of paradigm shifts to describe how scientific revolutions occur. A paradigm is a set of accepted practices and theories that define a scientific discipline at a particular time. When anomalies or inconsistencies within the current paradigm accumulate, they lead to a crisis, eventually resulting in a paradigm shift. This shift is a radical change in the underlying assumptions and methodologies of the field, leading to new ways of understanding and investigating phenomena.

One of the most controversial ideas in the book is the notion of incommensurability, which suggests that competing paradigms are often so different from one another that they cannot be directly compared. This means that the criteria for evaluating scientific theories can change with a paradigm shift, making it difficult to judge the superiority of one paradigm over another using the same standards. This idea challenges the notion of objective scientific progress and suggests that scientific knowledge is, to some extent, shaped by social and historical contexts.

Kuhn describes normal science as the routine work of scientists operating within an established paradigm. During periods of normal science, researchers focus on solving puzzles and refining theories within the existing framework. However, normal science is not a purely objective process; it is guided by the prevailing paradigm, which influences what questions are asked and what methods are used. This concept highlights the role of established scientific communities in shaping the direction of research and the acceptance of new ideas.