Taxes have been defined as "the enforced proportional contributions from persons and property levied by the state by virtue of its sovereignty for the support of the government and all public needs." The essential elements that we will notice particularly are two; first, that the contributions are proportional, that is, levied upon all in the same class according to some impartial standard, and second, that taxes can be levied for public purposes only.
Sage, Frank L. "Some Legal Aspects of Special Assessments." Mich. L. Rev. 2 (1904): 453-67.