One of you asked a terrific question after yesterday's class: If Article I assigns the legislative power to Congress (and to Congress alone), how can Congress delegate any of its legislative authority to administrative agencies (or anyone else) without violating the Constitution? The answer, I think, lies in distinguishing lawmaking in its technical, formal sense from "lawmaking" in a more practical, on-the-ground sense.
No doubt, only Congress has the formal constitutional authority to enact federal laws. At the same time, every time a federal law is implemented, some policymaking discretionary judgment is at play. Consider a very simple example (involving a state law, but the point is the same). Suppose a law prohibits anyone from driving an automobile "hazardously" or "recklessly" in a manner that "endangers public safety." A police officer sees a driver changing lanes continuously to move more quickly through traffic, often nearly clipping other cars. Does this conduct fall within the statute? Maybe. The police officer -- the person executing the law -- will have to make a discretionary judgment. And this judgment is effectively a policy judgment (even if not so intended by the officer). The same is true, of course, when the EPA establishes a standard of parts per million of soot for what limit is "requisite to protect the public health."
Indeed, virtually every act of law implementation, enforcement, or administration involves some judgment. And those judgments effectively make policy. They determine, in a very real sense, what the law means and how it will effectively govern human conduct. Do these discretionary judgments constitute "lawmaking"? Not in a technical sense; they are only permissible to the extent that they are authorized by the relevant statute. In promulgating its standard for soot, for instance, the EPA had to justify and explain its judgment according to 109(b)(1). It had to be a plausible implementation of the statute. Still, you can see why many people believe that these sorts of judgments amount to lawmaking in a practical sense.
This is essentially the difference between Justices Scalia and Stevens in Whitman. They are no different in their practical analyses of the legal issue: section 109(b)(1) of the Clean Air Act, at least interpreted to mean "sufficient, but not more than necessary," provides an "intelligible principle" to the EPA and thus is constitutional. But they differ on the semantic question of whether this is a delegation of discretion in enforcement (Scalia) or a delegation of lawmaking power (Stevens).