What does incommensurable mean?
Incommensurable is a formal adjective meaning that two things cannot be measured or assessed against one another through a common unit or standard. In mathematics, the term applies to quantities with no common measure, as with the side and diagonal of a square, whose ratio is irrational. In philosophical, ethical, and formal everyday usage, it describes values, goals, perspectives, or experiences so different that direct comparison is either impossible or misleading. The word therefore carries more than a claim of unequal size: it suggests that the items may not even belong to a shared scale. This makes it useful when discussing incompatible frameworks, such as competing conceptions of beauty, justice, or value. Its unusual sound and Latin ancestry also make it memorable, but its precision should guide its use. In most contexts, incommensurable is better suited to careful discussion than to casual description.
adjectiveUnable to be measured or compared using a common unit or standard; so different in nature, scale, or framework that direct comparison is impossible or misleading.
- Mathematics: Having no common measure, especially when the ratio of two quantities cannot be expressed as a ratio of integers.
- Figurative: So different in nature, scale, or framework that meaningful comparison by a common standard is impossible or misleading.
"The side and diagonal of a square are incommensurable because their ratio cannot be expressed as a ratio of integers."
"The side and diagonal of a unit square are incommensurable because their ratio is irrational."
"Their ideals of justice proved incommensurable, making a direct comparison between them largely meaningless."
Some quantities cannot share a yardstick, while some values may not belong on the same scale at all.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does incommensurable come from?
The word entered English by the 17th century from French incommensurable, ultimately from Latin incommensurabilis, meaning “not measurable together.” Its Latin form combines the negative prefix in- with commensurabilis, itself derived from com-, meaning “together,” and mensura, meaning “measure.” The related English words commensurate, commensurable, and commensurability preserve the same Latin foundation.
Related word forms
How incommensurable is actually used
This is primarily a formal, technical, or philosophical word. It is stronger than merely unequal and should generally be reserved for things that lack a common basis for comparison.
Easily confused with incommensurable
Incommensurable means lacking a common measure or basis for comparison, whereas incommensurate usually means unequal in size or not proportionate to something else.