A measurement is only useful if the other person means the same thing by it

The argument

The second argument for metric is about shared definition rather than arithmetic. A unit is a promise that two people describing the same quantity will produce the same number, and that promise depends on there being one definition rather than several. Proponents point out that metric units are defined by international agreement and realised the same way everywhere, so a figure written in one country can be read in another without annotation. Imperial measures, by contrast, carry a history of local variation and still differ in use: the pint, the gallon and the ton each name different quantities in different countries that use them, so the unit alone does not settle what was meant. Proponents argue that this is where the real cost of a divided world of measurement appears. Design files, parts, recipes, doses, shipping manifests and scientific results all cross borders, and each crossing either carries a conversion or rests on an assumption. A conversion that is done silently, or assumed rather than done, produces the kind of failure that is found afterwards rather than caught at the time, and keeping a second system alive guarantees a standing population of these translation points. The argument closes on practicality rather than principle. Proponents note that metric is already the system in which technical and scientific work is stated, including in countries that have never converted their public measures, which means metric is the system in which measurements travel. They conclude that whatever the merits of older units within their home trades, the better system is the one that lets a quantity mean the same thing to whoever receives it, and that is the one with a single shared definition.

Premises

Counter-arguments

Critics answer that ambiguity is a problem of labelling rather than of system choice, and that it is solved by stating which pint or which gallon is meant, as regulated trades already do. They add that the appeal to how widely metric is used is an argument from convention: it explains why metric is convenient, not why it is better, and the same reasoning would have favoured whichever system happened to be dominant. Where a population already shares an understanding of its own measures, critics argue, measurements inside that population are not ambiguous at all, and the cross-border case is handled at the boundary by the people whose job it is to handle it.

Rejecting the premises

[Rejecting P1] Shared meaning can be secured by stating which variant is in use, which is what regulated trade already does, so a single global definition is sufficient rather than necessary. [Rejecting P2] Variation between countries is a fact about particular units rather than about imperial as a system, and each of those units is defined precisely within the jurisdiction that uses it. [Rejecting P3] Border crossings are handled by specialists and by software built for the purpose, so the risk is concentrated where it is managed rather than spread through ordinary use. [Rejecting C] At most it follows that one shared system has advantages for exchange, not that metric is better for the people measuring inside their own.