To harness the full potential of units of measurement, we need some degree of consistency. Subjective measures contain useful knowledge, but this can be difficult to share, based as it is on an individual's experience. The historian of science Theodore M. Porter describes measurement as a 'technology of distance': a tool that uses shared rules to bridge differences of culture and geography, allowing for the exchange of information. Following this logic, if measurement is another sort of language, then just as with words, individual units need reliable definitions in order to communicate. In the earliest societies, these definitions might need to be shared no more widely than a single settlement, meaning that the definition can be no more complex than knowing that a unit was equal to a certain body part. As these communities grew in size, though, the inadequacies of this system would become evident. Imagine a disagreement over trade or tribute: it is a moment of conflict that demands greater consistency in measurement.