Maslahah al Mursalah, a

(plural: al masaalih al mursalah). A consideration which is proper and harmonious with the objectives of the Lawgiver; it secures a benefit or prevents a harm, but the Sharee`ah provides no indication as to its validity or otherwise. For example, the Companions decided to issue currency, to establish prisons, and to impose a tax on agricultural lands despite the fact that no textual authority could be found for these measures