Kuhn 3: Incommensurability

[koganbot]

"Incommensurability" is a metaphor. For the most part, what Kuhn calls "incommensurable" are words (though of course, when words don't match, the numbers you'd get from actually "measuring" what the words are supposed to designate would be suspect). Kuhn's metaphor compares words to geometry: The hypotenuse of an isosceles right triangle is incommensurable with its side in that there's no way you can measure one against another and get an integer. There's always some remainder. So, by analogy, certain crucial terms from Aristotle's physics are incommensurable with Newton's physics in that you can't find terms in the latter that can match up with those crucial terms from the former without there being a leftover, a remainder, a residue.

Here's a brief version (from the Preface to The Essential Tension, 1977) of what Kuhn said in more detail in "What Are Scientific Revolutions?":

Aristotle's subject was change-of-quality in general, including both the fall of a stone and the growth of a child to adulthood. In his physics, the subject that was to become mechanics was at best a still-not-quite-isolable special case. More consequential was my recognition that the permanent ingredients of Aristotle's universe, its ontologically primary and indestructible elements, were not material bodies but rather the qualities which, when imposed on some portion of omnipresent neutral matter, constituted an individual material body or substance. Position itself was, however, a quality in Aristotle's physics, and a body that changed its position therefore remained the same body only in the problematic sense that the child is the individual it becomes. In a universe where qualities were primary, motion was necessarily a change of state rather than a state.

So there's no way that Aristotle's concepts of position and motion can find a home in Newton's physics, where motion is a state, rather than a change in state, and position is not a quality, so change in position is not a quality.

I'd add to this the idea that a common measure between two things is often some third thing, e.g. you can compare the heat in Alaska and heat in Hawaii by seeing how high mercury rises in a tube in one place in comparison to the other. Metaphorically, I'll say that you can "measure" two competing theories by checking them against what we variously call the facts or the data or sense impressions or reality etc. So if my theory predicts that there will be a giraffe in the closet, and yours predicts there won't be a giraffe in the closet, we can figure out who's right by looking in the closet and seeing if there's a giraffe. And if your theory predicts that there's a square circle in the closet, we don't even have to look, since a square circle is impossible, and we know this by definition.** However, say that according to my theory, a body in motion stays in motion unless acted on by an outside force, and you say that that's as absurd as calling circles square, since you know that motion is asymmetrical change in quality that leads to a final end result, e.g., a stone falling towards its resting place in the center or a man returning from sickness to health.

Given those differences between my theory and yours, what can we look at to resolve them, or what definition can we consult? How can we tell just by looking at him that a man being restored to health is an example of motion, when you've already decided that it is and I've already decided that it isn't?

These are not rhetorical questions that are intended to lead you to answer "Nothing!" and "We can't!" Actually, I don't like the term "incommensurable" and wish that Kuhn had used some other, but that's for a later discussion. The question I'll leave you with is this: Kuhn has shown - decisively, to my mind - that there's no single transcendent measure that can decide certain issues, such as which concepts of motion and position to use. But does this mean that there is no way whatsoever to compare the differing concepts, no types of "measures" beyond those two grand transcendent measures: facts and definitions? After all, starting in the late middle ages, in a process that took several hundred years, one type of physics (the Aristotelian) evolved into and was displaced by another (the Newtonian), and people thought they had good reasons with every step on that road. Did they? Were they right? Or was their choice just a matter of personal taste? If so, why did Newton's ideas achieve such unanimity among physicists after Newton? And when Newton was finally overturned by quantum physics, why did every physicist after 1928 subscribe to quantum physics?* Compare to nonscientific disciplines, where there are competing schools and so forth and where the competition never seems to have a period of let-up.

*This isn't meant to imply that I have any idea if the disagreements now between string theorists and others amount to a break in unanimity in regard to basic terms. I'm guessing that the unanimity isn't broken, but I'm hardly the person to know.

**EDIT: Kuhn might well have objected to my using the phrase "by definition" here, since one reason for his use of the term "paradigm" is that he believed we learn how to use a term by using it (when a science is not undergoing a revolution the scientists model their subsequent uses of terms like, say, "force," "mass," etc. on how they were taught to use it in looking at examples or solving textbook problems or being given experiments to conduct, not by looking up the definitions of these terms, which are rarely adequate to tell you how to apply them). But I'm using the term "by definition" here idiomatically, where "by definition" and "by meaning" and "by how we use it" are synonymous, and none of those phrases are really explanatory.

Comments

[dubdobdee.livejournal.com]

(the reason i thought my post might count as a filibuster was that, even before we had got the the meat of your post, i was distractingly and nitpickingly questioning the content of the very first paragraph - which for your was expository and not at issue -- and possibly merely raising alarms that had no bearing on the overall argument you were planning to make: ie a filibuster really, a seperate fight on MY territory to stop us every getting to yours! in fact i suspect this is going to be a theme: in that i read kuhn through the lens of the history of mathematics, which as a as a discipline is significantly distinct as a discipline in terms of its relationship to language, to models, to measure, and possibly to argument by analogy and etc... we shall see!)

[koganbot]

(But I botched the first paragraph, and we wouldn't have known if my having botched it was or wasn't significant for my subsequent exposition if we hadn't looked at what it should have said had I not botched it. And the relevance or irrelevance of math to our discussion is something to be discovered, certainly; if math tends to have evolved differently from any of the sciences, this is something we'd want to know, since one of my questions is if or how Kuhn's ideas about the sciences can be applied to the nonsciences.)