In my view, the theoretical justification for Chevron is no different from the theoretical justification for those pre-Chevron cases that sometimes deferred to agency legal determinations. As the D.C. Circuit, quoting the First Circuit, expressed it: "The extent to which courts should defer to agency interpretations of law is ultimately 'a function of Congress' intent on the subject as revealed in the particular statutory scheme at issue.'" An ambiguity in a statute committed to agency implementation can be attributed to either of two congressional desires: (1) Congress intended a particular result, but was not clear about it; or (2) Congress had no particular intent on the subject, but meant to leave its resolution to the agency. When the former is the case, what we have is genuinely a question of law, properly to be resolved by the courts. When the latter is the case, what we have is the conferral of discretion upon the agency, and the only question of law presented to the courts is whether the agency has acted within the scope of its discretion-- i.e., whether its resolution of the ambiguity is reasonable. As I read the history of developments in this field, the pre-Chevron decisions sought to choose between (1) and (2) on a statute-by-statute basis. Hence the relevance of such frequently mentioned factors as the degree of the agency's expertise, the complexity of the question at issue, and the existence of rulemaking authority within the agency. All these factors make an intent to confer discretion upon the agency more likely. Chevron, however, if it is to be believed, replaced this statute-by-statute evaluation (which was assuredly a font of uncertainty and litigation) with an across-the-board presumption that, in the case of ambiguity, agency discretion is meant.
It is beyond the scope of these remarks to defend that presumption (I was not on the court, after all, when Chevron was decided). Surely, however, it is a more rational presumption today than it would have been thirty years ago-- which explains the change in the law. Broad delegation to the Executive is the hallmark of the modern administrative state; agency rulemaking powers are the rule rather than, as they once were, the exception; and as the sheer number of modern departments and agencies suggests, we are awash in agency "'expertise."' If the Chevron rule is not a 100% accurate estimation of modern congressional intent, the prior case-by-case evaluation was not so either--and was becoming less and less so, as the sheer volume of modern dockets made it less and less possible for the Supreme Court to police diverse application of an ineffable rule. And to tell the truth, the quest for the "genuine"' legislative intent is probably a wild-goose chase anyway. In the vast majority of cases I expect that Congress neither (1) intended a single result, nor (2) meant to confer discretion upon the agency, but rather (3) didn't think about the matter at all. If I am correct in that, then any rule adopted in this field represents merely a fictional, presumed intent, and operates principally as a background rule of law against which Congress can legislate.
If that is the principal function to be served, Chevron is unquestionably better than what preceded it. Congress now knows that the ambiguities it creates, whether intentionally or unintentionally, will be resolved, within the bounds of permissible interpretation, not by the courts but by a particular agency, whose policy biases will ordinarily be known. The legislative process becomes less of a sporting event when those supporting and opposing a particular disposition do not have to gamble upon whether, if they say nothing about it in the statute, the ultimate answer will be provided by the courts or rather by the Department of Labor.
Scalia, Judicial Deference to Agency Interpretation of Law, 1989 Duke L.J. 511, 516-517. Hmm. Perhaps you had to be there :)