Key Principle
Incommensurability is the thesis that successive paradigms differ along three dimensions simultaneously, making neutral adjudication impossible. First, they disagree about which problems are legitimate and which methods are acceptable (standards incommensurability). Second, formal derivations of old laws from new ones are spurious because the variables change meaning even when symbols remain identical (conceptual incommensurability). Third, paradigms redistribute the similarity sets through which scientists group objects, rewiring perception and meaning across the discipline (perceptual incommensurability). "The normal-scientific tradition that emerges from a scientific revolution is not only incompatible but often actually incommensurable with that which has gone before" (Ch. IX, p. 103).
Why This Matters
Without incommensurability, Kuhn's model collapses into a cumulative, rational-choice account of scientific progress. It is the mechanism that makes revolutions genuinely non-cumulative. If paradigms differed only in ontology, they could be compared on shared evidential ground. Because they disagree about the rules of the game itself -- what counts as a problem, a solution, and even a datum -- neutral adjudication becomes structurally impossible.
Critically, incommensurability was widely misread as implying theories cannot be compared at all and that there is no rational basis for preference. Kuhn explicitly rejected both conclusions. Concrete results can bridge the gap: "However incomprehensible the new theory may be to the proponents of tradition, the exhibit of impressive concrete results will persuade at least a few of them that they must discover how such results are achieved" (Introductory Essay, p. xxxii). The problem is not total incomparability but limited communication and non-algorithmic transitions.
In his later work, Kuhn reinterpreted incommensurability as analogous to biological speciation: revolutions are events in which one discipline splits into increasingly incomprehensible subdisciplines. This reframing transforms incommensurability from a troubling epistemological problem into a sociological observation about disciplinary proliferation, giving it "real content" detached from "pseudoquestions about theory choice" (Introductory Essay). Scientific progress proliferates outward into specialized niches -- Darwinian branching, not linear advance.
Good Examples
Newton vs. Einstein -- the spurious derivation (Ch. IX): Deriving Newtonian laws from Einsteinian ones under limiting conditions produces formally identical equations, but every variable still refers to Einsteinian concepts. "Newtonian mass is conserved; Einsteinian is convertible with energy. Only at low relative velocities may the two be measured in the same way, and even then they must not be conceived to be the same" (p. 102). To make the derived equations genuinely Newtonian requires redefining variables -- which is translation between incommensurable frameworks, not derivation. "The transition from Newtonian to Einsteinian mechanics illustrates with particular clarity the scientific revolution as a displacement of the conceptual network through which scientists view the world" (p. 102).
Newton and corpuscularism -- standards reversal (Ch. IX): Gravity as innate attraction "was an occult quality in the same sense as the scholastics' 'tendency to fall' had been" (p. 105). Newton's paradigm legitimized a type of explanation the preceding paradigm explicitly prohibited. Standards do not merely change -- they can reverse direction, undermining any linear progress narrative. Newton's legitimization of innate forces cascaded across fields: it made electrical action-at-a-distance speakable (Franklin), enabled chemical affinity theory, and ultimately supported Lavoisier's and Dalton's work (Ch. IX, p. 106).
Copernicus, Dalton, Lavoisier -- similarity-set redistribution (Postscript): Copernicus moved earth into the planet category; Dalton reclassified salts and alloys against a new element/compound distinction; Lavoisier transferred metals from compounds to elements. Because scientific terms are learned by ostension against these groupings, redistributing even a subset rewires meaning across the discipline. The communication breakdown is "prior to the application of the languages in which it is nevertheless reflected" -- not a terminological dispute fixable by stipulating definitions (Postscript).
Counterpoints
Incommensurability is localized, not total. Scientists share stimuli, neural apparatus, common history, and everyday vocabulary. Translation is possible: one can learn to render the rival's theory into one's own language and become a predictor of their behavior. Yet translation preserves the boundary -- one inhabits a bilingual position, not a merged one (Postscript).
Values provide shared (if underdetermining) ground. Kuhn endorses shared evaluative criteria -- accuracy, consistency, scope, orderliness, fruitfulness -- but argues they function as values, not algorithmic rules. "Neither can be convicted of a mistake. Nor is either being unscientific" when two scientists weight these criteria differently (Postscript). This blocks the false dichotomy between neutral algorithm and irrationalism.
The controversy was partly an artifact of logical empiricism. Under the empiricist framework, theoretical terms derive meaning solely from their theoretical context; change the theory, change the meaning. Without this presupposition, the problem of incommensurability loses much of its philosophical force, even as the practical communication difficulties remain real (Introductory Essay).
Key Quotes
"Each group uses its own paradigm to argue in that paradigm's defense." -- Thomas S. Kuhn, Chapter IX (p. 94)
"Like the choice between competing political institutions, that between competing paradigms proves to be a choice between incompatible modes of community life." -- Thomas S. Kuhn, Chapter IX (p. 94)
"This need to change the meaning of established and familiar concepts is central to the revolutionary impact of Einstein's theory." -- Thomas S. Kuhn, Chapter IX (p. 102)
"There is no neutral algorithm for theory-choice, no systematic decision procedure which, properly applied, must lead each individual in the group to the same decision." -- Thomas S. Kuhn, Postscript
Rules of Thumb
- When two frameworks seem to share vocabulary but generate persistent misunderstanding, suspect similarity-set redistribution rather than mere terminological confusion. The disagreement may be about how objects are categorized, not just what words mean.
- Distinguish translation from conversion: one can learn to speak both paradigm-languages without undergoing the gestalt shift that constitutes genuine adoption. Translation bridges incommensurability without collapsing it. One can acknowledge a rival theory's superiority yet continue working in one's own framework.
- Beware the "special case" argument: if restricting a theory to its confirmed domain makes it irrefutable, the same move immunizes every theory ever used successfully -- and eliminates the mechanism that produces scientific advance (Ch. IX, p. 99). "Is it really any wonder that the price of significant scientific advance is a commitment that runs the risk of being wrong?" (Ch. IX, p. 101).
- Paradigm debates are structurally circular: each paradigm satisfies its own criteria while failing the opponent's. Recognize that "there is no standard higher than the assent of the relevant community" (Ch. IX, p. 94) -- resolution comes through persuasion, not rule-governed adjudication.
- New paradigms can win despite losing explanatory power: Newton could not mechanically explain gravity; Lavoisier could not explain qualitative properties; Maxwell could not provide a mechanical ether. Paradigms succeed by redefining what requires explanation (Ch. IX).
Related References
- Crisis and the Response to Crisis - How anomalies produce the crisis conditions under which incommensurability becomes visible
- Revolutions as Changes of World View - Perceptual evidence for incommensurability at the observational level
- The Invisibility of Revolutions - How textbooks disguise incommensurability by rewriting history as cumulative
Sources: Distilled chunks 003, 007, 029, 030, 031, 049, 050