<P> The authority to purchase is not one of the explicitly enumerated powers given to the Federal Government by Section 8 of Article One of the United States Constitution , but courts found that power implicit in the Constitutional power to make laws that are necessary and proper for executing its specifically granted powers , such as the powers to establish post offices , post roads , banks , an army , a navy , or militias . </P>
who decides which laws are necessary and proper in the u.s. system of government
Federal Government