What is "commensurability"

Commensurability \Com*men`su*ra*bil"i*ty\, n. [Cf. F. commensurabilit['e].] The quality of being commensurable.
--Sir T. Browne.

n. The quality of being commensurable or commensurate

In mathematics, two non- zero real numbers a and b are said to be commensurable if is a rational number.

''This article is about incommensurability in the philosophy of science. For other senses of this word, see commensurability (disambiguation). ''

Commensurability is a concept, in philosophy of science, whereby scientific theories are commensurable if scientists can discuss using a shared nomenclature that allows direct comparison of theories to determine which theory is more valid or useful. On the other hand, theories are incommensurable if they are embedded in starkly contrasting conceptual frameworks whose languages do not overlap sufficiently to permit scientists to directly compare the theories or to cite empirical evidence favoring one theory over the other. Discussed by Ludwik Fleck in the 1930s, and popularized by Thomas Kuhn in the 1960s, the problem of incommensurability results in scientists talking past each other, as it were, while comparison of theories is muddled by confusions about terms, contexts and consequences.

Two concepts or things are commensurable if they are measurable or comparable by a common standard.

Commensurability commonly refers to Commensurability (mathematics). It may also refer to:

• Commensurability (astronomy)
• Commensurability (economics), whether economic value can always be measured by money
• Commensurability (ethics), the commensurability of values in ethics
• Commensurability (law)
• Commensurability (philosophy of science)
• Unit commensurability, a concept in dimensional analysis that concerns conversion of units of measurement

it:Incommensurabilità simple:Incommensurability

In ethics, two values (or norms, reasons, or goods) are incommensurable when they do not share a common standard of measurement.

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?" On one popular view, answers to this question can be found 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 they should stay in to do work or go for a walk, on this view of practical reason they will compare the merits of these two options. If going for a walk is the better or more reasonable course of action, they should put aside their books and 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.

Commensurability is the property of two orbiting objects, such as planets, satellites, or asteroids, whose orbital periods are in a rational proportion.

Examples include the 2:3 commensurability between the orbital periods of Neptune and Pluto, the 3:4 commensurability between the orbital periods of the Saturnian satellites Titan and Hyperion, the orbital periods associated with the Kirkwood gaps in the asteroid belt relative to that of Jupiter, and the 2:1 commensurability between Gliese 876 b and Gliese 876 c.

Commensurabilities are normally the result of an orbital resonance, rather than being due to coincidence.

Commensurability in economics arises whenever there is a common measure through which the value of two entities can be compared.

Commensurability has two versions:

• Strong commensurability arises when it is possible to give cardinal values to entities as a consequence of utilising a given property measure entities. Thus we can say "This is two and a half times more valuable than that." This implies value monism.
• Weak commensurability arises when it is only possible to apply ordinal values to entities as a consequence of utilising a given property to rank entities, i.e., it is sufficient to say "This is more valuable than that." This is consistent with value-pluralism.

While weak commensurability is a form of strong comparability, it is distinct from weak comparability, where the fact that a comparison is valid in one context does not imply that it is so in all contexts. Also issues of comparability are different from indeterminacy: it may not be possible in certain circumstances to make a measurement, even though if such data was available it would be valid to compare measurements.

Commensurability is a key factor in the Socialist calculation debate.