We must now ask how changes of this sort can come about, considering first discoveries, or novelties of fact, and then inventions, or novelties of th… - Thomas Kuhn

Somehow, the practice of astronomy, physics, chemistry or biology normally fails to evoke the controversies over fundamentals that today seem endemic among, say, psychologists or sociologists. Attempting to discover the source of that difference led me to recognize the role in scientific research of what I have since called “paradigms.” These I take to be universally recognized scientific achievements that for a time provide model problems and solutions for a community of practitioners.

Out-of-date theories are not in principle unscientific because they have been discarded. That choice, however, makes it difficult to see scientific development as a process of accretion.

If a demarcation criterion exists (we must not, I think, seek a sharp or decisive one), it may lie just in that part of science which Sir Karl ignores.