Kuhn's notion of "incommensurability"

Thomas Kuhn, a philosopher of science, who popularized the term "paradigm shift" i.e. there are periods of conflict in history when a paradigm (model) fails to provide adequate models for phenomenon, a new and more powerful explanatory force or a paradigm shift takes place. Some identified paradigm shift in history which created new paradigms or a revolution in thinking is Copernicus' heliocentric theory which replaced the Ptolemaic theory that the sun revolves around the earth. Also Einstein's replacement of Newton's theory of gravity and motion.

Kuhn also popularized the notion of 'incommensurability' which defied the notion that science is on an advancing path of progress towards ultimate truth. According to Kuhn, the rejection of a previous paradigm in favour of a completely different one rules out the possibility of comparison. Kuhn argues that the scientists view of the world is so radically altered by the acceptance of a new paradigm that the old and new are quantitatively and qualitatively incomparable. Kuhn argues that scientists operating at different historical periods with different paradigms live in a psychologically different worlds.

Kuhn says that after Copernicus, astronomers lived in a different worlds. His idea of the world is not the same world as Copernicus, for when Ptolemy observes the sun he observes an object that movesa round the earth, whereas Copernicus sees the central star of the solar system.

This subjectivism in science makes the idea of absolute truth a questionable notion, and according to Kuhn, one we can do without. Since it is impossible to investigate the nature of reality without operating with some paradigm, or other, we should see science as the evolution of ideas in response to the world.

Kuhn compared the evolution of an organism to an evolution of ideas towards some ultimate truths. (Philosophy 100 essential thinkers)