The Structure of Scientific Revolutions was soon criticized by Kuhn's colleagues in the history and philosophy of science. In 1965, a special symposium on the book was held at an International Colloquium on the Philosophy of Science that took place at Bedford College, London, and was chaired by Karl Popper. The symposium led to the publication of the symposium's presentations plus other essays, most of them critical, which eventually appeared in an influential volume of essays. Kuhn expressed the opinion that his critics' readings of his book were so inconsistent with his own understanding of it that he was "tempted to posit the existence of two Thomas Kuhns", one the author of his book, the other the individual who had been criticized in the symposium by "Professors Popper, Feyerabend, Lakatos, Toulmin and Watkins.[49]
A number of the included essays question the existence of normal science. In his essay, Feyerabend suggests that Kuhn's conception of normal science fits organized crime as well as it does science.[50] Popper expresses distaste with the entire premise of Kuhn's book, writing, "the idea of turning for enlightenment concerning the aims of science, and its possible progress, to sociology or to psychology (or ... to the history of science) is surprising and disappointing."[51]
Concept of paradigm
Stephen Toulmin defined paradigm as "the set of common beliefs and agreements shared between scientists about how problems should be understood and addressed". In his 1972 work, Human Understanding, he argued that a more realistic picture of science than that presented in The Structure of Scientific Revolutions would admit the fact that revisions in science take place much more frequently, and are much less dramatic than can be explained by the model of revolution/normal science. In Toulmin's view, such revisions occur quite often during periods of what Kuhn would call "normal science". For Kuhn to explain such revisions in terms of the non-paradigmatic puzzle solutions of normal science, he would need to delineate what is perhaps an implausibly sharp distinction between paradigmatic and non-paradigmatic science.[52]
Incommensurability of paradigms
In a series of texts published in the early 1970s, Carl R. Kordig asserted a position somewhere between that of Kuhn and the older philosophy of science. His criticism of the Kuhnian position was that the incommensurability thesis was too radical, and that this made it impossible to explain the confrontation of scientific theories that actually occurs. According to Kordig, it is in fact possible to admit the existence of revolutions and paradigm shifts in science while still recognizing that theories belonging to different paradigms can be compared and confronted on the plane of observation. Those who accept the incommensurability thesis do not do so because they admit the discontinuity of paradigms, but because they attribute a radical change in meanings to such shifts.[53]
Kordig maintains that there is a common observational plane. For example, when Kepler and Tycho Brahe are trying to explain the relative variation of the distance of the sun from the horizon at sunrise, both see the same thing (the same configuration is focused on the retina of each individual). This is just one example of the fact that "rival scientific theories share some observations, and therefore some meanings". Kordig suggests that with this approach, he is not reintroducing the distinction between observations and theory in which the former is assigned a privileged and neutral status, but that it is possible to affirm more simply the fact that, even if no sharp distinction exists between theory and observations, this does not imply that there are no comprehensible differences at the two extremes of this polarity.
At a secondary level, for Kordig there is a common plane of inter-paradigmatic standards or shared norms that permit the effective confrontation of rival theories.[53]
In 1973, Hartry Field published an article that also sharply criticized Kuhn's idea of incommensurability.[54] In particular, he took issue with this passage from Kuhn:
Newtonian mass is immutably conserved; that of Einstein is convertible into energy. Only at very low relative velocities can the two masses be measured in the same way, and even then they must not be conceived as if they were the same thing.
— Kuhn (1970)
Field takes this idea of incommensurability between the same terms in different theories one step further. Instead of attempting to identify a persistence of the reference of terms in different theories, Field's analysis emphasizes the indeterminacy of reference within individual theories. Field takes the example of the term "mass", and asks what exactly "mass" means in modern post-relativistic physics. He finds that there are at least two different definitions:
- Relativistic mass: the mass of a particle is equal to the total energy of the particle divided by the speed of light squared. Since the total energy of a particle in relation to one system of reference differs from the total energy in relation to other systems of reference, while the speed of light remains constant in all systems, it follows that the mass of a particle has different values in different systems of reference.
- "Real" mass: the mass of a particle is equal to the non-kinetic energy of a particle divided by the speed of light squared. Since non-kinetic energy is the same in all systems of reference, and the same is true of light, it follows that the mass of a particle has the same value in all systems of reference.
Projecting this distinction backwards in time onto Newtonian dynamics, we can formulate the following two hypotheses:
- HR: the term "mass" in Newtonian theory denotes relativistic mass.
- Hp: the term "mass" in Newtonian theory denotes "real" mass.
According to Field, it is impossible to decide which of these two affirmations is true. Prior to the theory of relativity, the term "mass" was referentially indeterminate. But this does not mean that the term "mass" did not have a different meaning than it now has. The problem is not one of meaning but of reference. The reference of such terms as mass is only partially determined: we do not really know how Newton intended his use of this term to be applied. As a consequence, neither of the two terms fully denotes (refers). It follows that it is improper to maintain that a term has changed its reference during a scientific revolution; it is more appropriate to describe terms such as "mass" as "having undergone a denotional refinement".[54]
In 1974, Donald Davidson objected that the concept of incommensurable scientific paradigms competing with each other is logically inconsistent.[55] In his article Davidson goes well beyond the semantic version of the incommensurability thesis: to make sense of the idea of a language independent of translation requires a distinction between conceptual schemes and the content organized by such schemes. But, Davidson argues, no coherent sense can be made of the idea of a conceptual scheme, and therefore no sense may be attached to the idea of an untranslatable language."[56]
Incommensurability and perception
The close connection between the interpretationalist hypothesis and a holistic conception of beliefs is at the root of the notion of the dependence of perception on theory, a central concept in The Structure of Scientific Revolutions. Kuhn maintained that the perception of the world depends on how the percipient conceives the world: two scientists who witness the same phenomenon and are steeped in two radically different theories will see two different things. According to this view, our interpretation of the world determines what we see.[57]
Jerry Fodor attempts to establish that this theoretical paradigm is fallacious and misleading by demonstrating the impenetrability of perception to the background knowledge of subjects. The strongest case can be based on evidence from experimental cognitive psychology, namely the persistence of perceptual illusions. Knowing that the lines in the Müller-Lyer illusion are equal does not prevent one from continuing to see one line as being longer than the other. This impenetrability of the information elaborated by the mental modules limits the scope of interpretationalism.[58]
In epistemology, for example, the criticism of what Fodor calls the interpretationalist hypothesis accounts for the common-sense intuition (on which naïve physics is based) of the independence of reality from the conceptual categories of the experimenter. If the processes of elaboration of the mental modules are in fact independent of the background theories, then it is possible to maintain the realist view that two scientists who embrace two radically diverse theories see the world exactly in the same manner even if they interpret it differently. The point is that it is necessary to distinguish between observations and the perceptual fixation of beliefs. While it is beyond doubt that the second process involves the holistic relationship between beliefs, the first is largely independent of the background beliefs of individuals.
Other critics, such as Israel Scheffler, Hilary Putnam and Saul Kripke, have focused on the Fregean distinction between sense and reference in order to defend scientific realism. Scheffler contends that Kuhn confuses the meanings of terms such as "mass" with their referents. While their meanings may very well differ, their referents (the objects or entities to which they correspond in the external world) remain fixed.[59]
