The laws have one point in common with the so-called natural laws; both are concerned with empirical concepts, or classes. But while the natural laws are mere indicative statements of fact, the laws can always be translated from the indicative to the imperative; that is, they contain a volitive element which is absent from the natural laws. The volitive element is present, on the other hand, in the practical principles which have some time received the names of moral or economic laws, and which can be converted into such imperatives as Will the universal, or, in particular, Will the good, the useful, the true, the beautiful. But these principles are concerned with the universal, that is, with the spirit of man in the necessary forms of its activity, not with a particular product of the spirit, a class or type of actions, as do laws in the strict meaning of the word. This distinction between the practical principles and the laws opens the way to the recognition of a very important character of the laws: while the practical principles, because of their universality, have no limits and no exceptions (and we have already seen that a morally indifferent action is a contradiction in terms), the laws can never exhaust the universal, and therefore will always leave outside themselves a margin of actions, not included in any of the classes to which they refer, and therefore legally indifferent. In more technical language, we may express the same idea by saying that all laws, whether imperative or prohibitive or permissive (a law, according to the ancient formula, _aut iubet aut vetat aut permittit_), can be reduced to permissive laws: an order is always at the same time a prohibition, and both orders and prohibitions implicitly permit all actions which are not contemplated by the law.