Also, some matters are by their very nature impossible to categorize under a single head of power: they may have both provincial and federal aspects. The double aspect doctrine, which applies in the course of a pith and substance analysis, ensures that the policies of the elected legislators of both levels of government are respected, by recognizing that both Parliament and the provincial legislatures can adopt valid legislation on a single subject depending on the perspective from which the legislation is considered (i.e., depending on the various aspects of the matter in question).