Affirmative authorization

What does Affirmative authorization mean in American Law?

The definition of Affirmative authorization in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

Frequently used to describe an authorization (see authorize) explicitly or particularly given, as contrasted with one given implicitly or generally. Thus, if a statute said “Any landowner may build a dam on a navigable stream on his land,” that would clearly be an “affirmative authorization” for dam building. But if there were no statute specifically in point (and thus the authorization, if any, would have to be inferred from silence against a background of ordinary property rights), or if the statute merely said “landowners may use navigable streams on their land,” then one would most likely not describe a dam as “affirmatively authorized.”