Knowability is the ability to identify a preferable course of action with sufficient confidence to justify adopting that course. This article shows it is not possible to judge the value of a public value proposition with sufficient confidence to justify the use of public authority. The indeterminacy of public value is shown by demonstrating that the necessary conditions to justify a public value proposition include that the evidence sustaining it is not impossible, circular, or unsubstantiated opinion. Those criteria are applied to an exhaustive set of possible concepts of public value, all of which fail at least one of those conditions so public value is unknowable. The implication is not that government is impossible, but that it requires humility, discourse, and compromise.