Generally, two quantities are commensurable if both can be measured in the same unit of measurement. For example, a distance measured in miles and a volume of water measured in gallons are incommensurable. On the other hand, time measured in weeks and time measured in minutes are commensurable because a week is a constant number of minutes (10080), so that one can convert between the two units by multiplying or dividing by 10080.
In the philosophy of science, two theories are said to be incommensurable if there is no common theoretical language that can be used to compare them. If two scientific theories are incommensurable, there is no way in which one can compare them to each other in order to determine which is better. In ethics, two values (or norms, reasons, or goods) are incommensurable when they do not share a common standard of measurement.
The concept of incommensurability became an issue when Thomas Kuhn introduced the concept of paradigm in his The Structure of Scientific Revolutions. Kuhn argued that a shift from one paradigm to another is not a linear progress but a radical change (see Scientific Revolution) of incommensurable paradigms, which can be comparable to religious conversion. The concept of incommesurability was applied beyond the philosophy of science to ethics, and it was used to explore the possibility of comparing competing ethical theories.
The concept of incommensurability also raises questions regarding translation. Each language is configured and structured by its own unique paradigm constituted by several elements such as cultural norms, people's experiences, their unique history and characteristics. Then, in order to understand what is meant by another language one may have to understand its social, historical, and cultural contexts. However, if languages are incommensurable and do not share any common ground, then translation cannot be more than a rough approximation of what is meant. This question of incommensurability raises questions regarding the interaction between different cultures, traditions, and value perspectives and is an important topic in today's global multicultural environment.
The idea that scientific paradigms are incommensurable was popularized by the philosopher and historian of science Thomas Kuhn in his book The Structure of Scientific Revolutions (1962). He wrote that when paradigms change, the world itself changes with them (see esp. Chapter X of this book). According to Kuhn, the proponents of different scientific paradigms cannot make full contact with each other's point of view because they are, as a way of speaking, living in different worlds. Kuhn gave three reasons for this inability:
In a postscript (1969) to The Structure of Scientific Revolutions, Kuhn added that he thought that incommensurability was, at least in part, a consequence of the role of similarity sets in normal science. Competing paradigms group concepts in different ways with different similarity relations. According to Kuhn, this causes fundamental problems in communication between proponents of different paradigms. It is difficult to change such categories in one's mind, because the groups have been learned by means of exemplars instead of definitions. This problem cannot be resolved by using a neutral language for communication, since the difference occurs prior to the application of language.
The philosophy of Paul Feyerabend was also based on the idea of incommensurability to a large extent. Feyerabend argued that frameworks of thought and thus scientific paradigms can be incommensurable for three reasons. His list of reasons is similar to that of Kuhn. However, Feyerabend first presented his notion of incommensurability in 1952 to Karl Popper's LSE seminar. Included in the group was Elizabeth Anscombe, Peter Geach, H.L.A. Hart and Georg Henrik von Wright. Briefly put, Feyerabend's notion of incommensurability is as follows:
According to Feyerabend, the idea of incommensurability cannot be captured in formal logic, because it is a phenomenon outside of its domain.
Donald Davidson criticized the notion of incommensurability in an article entitled On the very idea of a conceptual scheme.
Davidson's critique is aimed at conceptual relativism—the idea that reality is relative to a scheme, and hence that what is real in one scheme may not be real in another.
Davidson proceeds by pointing out that "where conceptual schemes differ, so do languages." That is, that to hold to a particular conceptual scheme is to hold to a particular language. It follows then that two conceptual schemes would be incommensurable only in the case that it was not possible to translate the theory expressed in the language of one scheme into the ideas expressed in the language of another. He argues that it is impossible to make sense of a total failure to be able to translate a given theory from one language to another. From this it follows that it is impossible to make sense of the notion of two theories being incommensurable.
Davidson's notion that holding onto a particular conceptual scheme is to hold to a particular language closely parallels the much earlier writings of Ludwig Wittgenstein (1889 - 1951). Wittgenstein argued that our communication can be understood as a series of 'language games', in which it is a mistake to take things that sound alike (what we would call the 'same words') from one game, and use them in another game. These individual games are, for Wittgenstein, incommensurable.
To understand Wittgenstein's position, it is necessary to clarify his concept of "family resemblance." In Philosophical Investigations, Wittgenstein examined the question of why and how all kinds of games are called "games." Previous philosophers of language presupposed that, if a word is used in a certain way, a word should have some characteristics, denominators, or essence common to all items to which it is applied. Wittgenstein, however, denied this thesis. He argued that all kinds of games are called games not because they have some common characteristics, but because they are more or less similar. Just like all members of the same family more or less look alike, all games are called as such because of their loose similarity. Wittgenstein further argued that the meaning of a word is not defined by some kind of essence or properties, but by how the word is "used" in a given context.
Thus, Wittgenstein argued that the meaning of the term or concept is highly contextualized; two different contexts, be it a theory or a culture, can be incommensurable although they may present a loose similarity.
The concept of incommensurability has been introduced to the philosophical analysis of the possibility of translation. Each language has been configured by its unique paradigm which is constituted by a number of factors such as people's experience, characteristics, norms, value perspectives, history, culture, and others. For example, people's experiences and vocabulary in one linguistic community are not necessarily the same as those of another linguistic community.
As Wittgenstein argued, if the meaning of words are determined by how they are used in a given language, and language is configured by its cultural contexts, one must understand the cultural contexts themselves in order to understand the meaning of the work in a given language. Translation cannot be more than very loose approximation of meaning.
We can see a number of examples. For example, when Aristotle's ancient Greek texts were translated into Latin, the original ideas were radically distorted. For this reason, understanding a foreign language, be it ancient or contemporary language, requires the understanding of its incommensurable paradigm consituted by cultural norms, values, and history.
Philosophers argue over the precise nature of value incommensurability, and discussions do not always exhibit a consistent terminology. It is frequently said that two values are incommensurable if and only if, when compared, neither is better than the other nor are they equally valuable. This result does not follow from the strict definition of incommensurability (absence of a common standard of measurement). Thus some prefer to use "incommensurable" when describing pairs that lack a common measure and to use the word "incomparable" more specifically when describing incommensurable pairs containing members neither of which is better than or equal to the other.
Philosophical reflection about practical reason typically aims for a description of the principles relevant in answering the question, "What is to be done in this or that circumstance?" One popular view answers this question by comparing the relative strengths of the various values or norms in play in some given situation. For example, if one is trying to decide on some nice afternoon whether to stay in to do work or go for a walk, one would compare the merits of these two options. If going for a walk is the better or more reasonable course of action, one should go for a stroll. The topic of incommensurability—and the topic of incomparability in particular—is especially important to those who advocate this view of practical reason. For if one's options in certain circumstances are of incomparable value, he or she cannot settle the question of what to do by choosing the better option. When the competing options are incomparable, then by definition neither is better than the other.
In recent decades, incommensurability has figured prominently in recent philosophical debates over the possibility of moral dilemmas and the plausibility of certain forms of consequentialism in ethics. The incommensurability of various types of moral reason is often seen as explaining how moral dilemmas and other ethical conflicts might be possible. Incommensurability also presents a prima facie challenge to ethical theories that contend that the right thing to do is the action that promotes the most overall good; if value incommensurability is widespread enough to make most values incommensurable with one another, then it seems that the utilitarian calculus is not even theoretically possible.
The topic of incommensurability has also frequently arisen in discussions of the version of natural law theory associated with John Finnis and others.
The question of commensurability arises when we consider the intricate relationship between scientific knowledge and religious knowledge; each type of knowledge seems to have, among many other characteristics, a discrete epistemological methodology, concept of truth, and linguistic expression.
While religious knowledge relies on revelation, spiritual experience, and tradition, science relies on sense experience and physically detectable data. Furthermore, the concept of truth in religion and science are considerably different. Understanding of religious truth often requires its embodiment or existential changes in the practitioner, while scientific truth is conceptual and objective and the observer is uninvolved in the object of cognition.
Thomas Kuhn argued that scientific knowledge is not a-historical, objective, and free of interpretation. Rather, he argues that scientific knowledge is deeply rooted in its social, historical, and hermeneutic elements. Kuhn conceptualized the basic framework of interpretation, held by scientific communities, as "paradigm." Through the study of various philosophies of hermeneutics, Kuhn realized the affinity of his concept of paradigm with the idea of "hermeneutic basis" in philosophic hermeneutics. From scriptural readings to revelations, hermeneutics is also central to religious knowledge. Thus, the key to the question of commensurability between scientific and religious knowledge may be philosophic hermeneutics. As Heidegger notes, the fundamental principle of human existence may be already hermeneutic.
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