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This article is about commensurability in ethics. For other senses of the word, see commensurability.

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 I am trying to decide on some nice afternoon whether I should stay in to do work or go for a walk, on this view of practical reason I will compare the merits of these two options. If going for a walk is the better or more reasonable course of action, I should put aside my 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 (philosophy of science)

• Chang, Ruth (editor). Incommensurability, Incomparability, and Practical Reason. Cambridge: Harvard University Press, 1997.
• Finnis, John. Natural Law and Natural Rights. Oxford: Clarendon Press, 1980.
• Raz, Joseph. The Morality of Freedom. Oxford: Clarendon Press, 1986.