Many people know that the philosopher of science Thomas Kuhn was often accused of encouraging “mob psychology” in science. (This accusation dates back to the words of Imre Lakatos.) Kuhn was said to have rejected objectivity and rationality in science too. In this essay, it’s shown that the philosopher of science Ian Hacking defended Kuhn from these accusations. However, he might have been guilty of going too far in the opposite direction.
Kuhn on Incommensurability
According to the philosopher and historian of science Ian Hacking, Thomas Kuhn believed that “[y]ou don’t have two theories in mind and compare them point by point — they are too different for that”. So if they’re too different to compare, then what happens? Perhaps what happens is what has already happened. A scientist has already placed his bet on one of the rival theories. But why did he do so in the first place? Kuhn believed that he “gradually convert[ed], and that shows itself by moving into a new language community”. Then such a scientist “begin[s] to speak the language like a native”, yet no “choice has occurred”.
The way all the above is put makes it seem as if the process is almost purely sociological in nature. So it’s not that two theories are compared point by point. A scientist moves into a new language community, and only then does he choose a theory. And the theory he chooses is expressed in the language of the new community. Other rival theories are expressed in the languages of other communities. It may not even be the case that a scientist chooses the language community, let alone which scientific theory to adopt or endorse.
Again, this process seems entirely sociological (as well as psychological) in nature.
What begins as a sociological process becomes a historical process in retrospect. And that’s another lesson from Kuhn. Hacking put it bluntly:
“The discourse of the philosophy of science has been transformed since the time Kuhn wrote. No longer shall we, as Nietzsche put it, show our respect for science by dehistoricizing it.”
Hacking provides the example of Larry Lauden, who “draws his conclusion ‘from the existing historical evidence’”.
Note that the first quote states that “philosophy of science has been transformed since the time Kuhn wrote”. It doesn’t say that the philosophy of science was transformed by what Kuhn wrote… at least he wasn’t the only man in the team. According to Hacking, “When his book The Structure of Scientific Revolutions came out in 1962, similar themes were being expressed by a number of voices.” Indeed, in 1962 and just before, “a new discipline, the history of science, was forming itself”. Moreover, “[y]oung Kuhn, training as a physicist, was attracted to history just at the moment when many other people were looking that way”.
The point about scientific rationality is that it’s linked, or can be linked, to the subject of incommensurability. Hacker supplies some basic historical details about this term’s use:
“The new philosophical use of the word ‘incommensurable’ is the product of conversations between Paul Feryerabend and Thomas Kuhn on Berkeley’s Telegraph Avenue around 1960.”
So what does the word “incommensurable” mean?
“It has an exact sense in Greek mathematics. It means ‘no common measure’. [ ] Not all lengths are commensurable.”
More relevantly:
“Philosophers have nothing so precise in mind when they use the metaphor of incommensurability. They are thinking of comparing scientific theories, but of course there could be no *exact* measure for that purpose.”
The word “incommensurable” certainly sounds forceful. Moreover, that word has led to many philosophical and political consequences. Yet, in Greek mathematics the term was precise. Not so in philosophy. Hacking says the word is a “metaphor”. Yes, it’s a metaphor that’s done a hell of a lot of good or bad work.
When used to describe scientific theories, Hacking concedes that “there could be no exact measure for that purpose”. From how Hacking phrases it both here and elsewhere, this isn’t a monumental fact about scientific theories. Instead, it’s rather mundane. Think about it, both in the abstract and in concreto, what form would such a measure take? Even if there were such a measure, would it apply to all aspects of all theories when measured one-to-one or one-to-many?
Precisely because the philosophical word “incommensurable” is both a metaphor and imprecise, it’s no surprise that the (possible) anecdote that Kuhn used the word in 21 different ways (in his well-known book) did the rounds so often. (See here.)
So we can respect science and scientists and still place them within their historical contexts.
Was Kuhn Against Scientific Rationality?
Many commentators took Kuhn to have questioned scientific rationality. Some commentators even believed that was his main aim. Perhaps relevantly, they questioned why he used the strong word “revolution” (as in “scientific revolution”).
According to Hacking, Kuhn was “taken aback by the way in which his work (and that of others) produced a crisis of rationality”. Of course, even if Kuhn was taken aback, that doesn’t mean that his work didn’t produce a crisis of rationality. Perhaps he simply didn’t think through the consequences of his own theories, and how they may be interpreted.
The following description of what Kuhn believed is an attempt to show that he didn’t reject rationality:
“He subsequently wrote that he never intended to deny the customary virtues of scientific theories. Theories should be accurate, that is, by and large fit existing data. They should be both internally consistent and consistent with other accepted theories. They should be broad in scope and rich in consequences. They should be simple in structure, organising facts in an intelligible way. They should be fruitful, disclosing new events, new techniques, new relationships.”
That paragraph almost reads like something you’d come across in an introductory book on the philosophy of science. In fact, at first sight, the words “consistent with other accepted theories” seem to go against Kuhn’s known views.
All the above now needs to be placed in the context that Kuhn believed that “his five values and others of the same sort are never sufficient to be decisive among competing theories” too. So what, in addition to Kuhn’s five values, would make a theory win out?
The biologist and parapsychology researcher Rupert Sheldrake, who mentions and quotes Kuhn a few times, makes much of the fact that “scientists are people”. Yet science itself isn’t a person. Perhaps we can see science as being made up of people. However, even if we take science as a combination of people, that collective is still not a single person.
Hacking also says that “scientific societies are societies”. One would assume, then, that if sociology is the study of society, then it must study scientific societies too. Does that mean it should study science itself? Does that question tie into the earlier one about science being made up of people?
Conclusion
Kuhn is known for using the case of Aristotle to get his point across. According to Hacker’s Kuhn, the latter tells us that
“Aristotle’s physics relies on ideas of motion that are dissociated from ours, and one can understand him only by recognising the network of his words”.
This passage isn’t that clear, especially the use of the words “dissociated from ours”. However, the words “the network of his words” can be tied to the earlier use of “language community”. In basic terms, Aristotle belonged to his own language community, not ours. Kuhn had much more to say about stepping into Aristotle’s world in order to understand him. (Historians often say something very similar about historical characters, ideas and events.) Thus, Hacker concluded:
“Kuhn is one of many historians to teach the need to rethink the words of our predecessors in their own way, not ours.”
Writing in 2026, it seems remarkable that any philosopher, scientist or political activist could ever have thought otherwise. That said, present-day presentism has never been so rife. Of course, rethinking Aristotle or anyone else doesn’t mean that we should accept his own way. Indeed, we should neither accept nor reject it (at least not judgementally or even morally). Instead, we can, or should, learn from it. Kuhn himself learned that the words and theories of scientists are historically and contextually bound. Yet even if that is acknowledged, some philosophers of science believe that the matters of truth and correctness are still relevant, and even important. As earlier, just as science being carried out by flesh-and-blood scientists doesn’t make science itself suspect, so the words and theories of scientists being historically, sociologically and contextually bound doesn’t automatically make such words and theories suspect or false. It is up to philosophers of science, scientists (less so) and others to sort the wheat from the chaff.
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