During the nineteenth century, law was equated with science, and legal reasoning was thought to be a species of deductive logic. Consistent with this notion, judicial opinions have traditionally been summarized in the form of syllogisms, that is, as arguments of deductive logic. More specifically, judicial opinions have been described as chains of syllogisms, reasoning from base premises to ultimate conclusions. The principal thrust of this article is to demonstrate that in hard cases, judicial reasoning proceeds not by way of deduction, but by evaluation and balancing.
Accordingly, Part II of this article compares law with science. Historically, law was considered a science and was thought to have the same underlying structure. Both law and science consist of sets of rules; both are concerned with predicting events; both legal reasoning and scientific reasoning use a framework of deductive logic to reason from general principles to particular results; and both legal principles and scientific principles evolve over time.
Over the last century, however, legal scholars have rejected the identification of law with science. The principal difference between law and science is that while science is based upon and must be reconciled with objective observations of nature, law arises from value judgments.
Part III uses the structure of the syllogism to explain the difference between easy cases and hard cases. Easy cases are governed by unambiguous legal rules of unchallenged validity; in such cases, the legal conclusion follows deductively from the applicable rule of law. However, as several scholars have noted, there are two kinds of hard cases: cases where the applicable rule of law is ambiguous in relation to the facts of the case (questions of ambiguity), and cases where it is uncertain what the applicable rule of law is (questions of validity). Ultimately, all hard cases, both questions of ambiguity and questions of validity, are resolved in the same manner, by resort to the fundamental categories of legal argument. This exposes a second fundamental difference between law and science: while science is grounded in a single source (observations of the physical universe), law springs from multiple sources (text, intent, precedent, tradition and policy), which often give rise to legitimate but conflicting interpretations of the law.
Part IV describes the use of the syllogism in analyzing judicial opinions. Case briefs are not mere syllogisms?they are chains of syllogisms ("polysyllogisms"), in which the conclusions of syllogisms earlier in the chain supply the premises of syllogisms that are later in the chain. Questions of ambiguity arise when the minor premise of a legal argument is challenged, while questions of validity represent a challenge to the major premise of a legal argument. At the base of each chain of syllogisms are premises about the law. The base major premises of each chain of legal reasoning consist of the five types of legal argument, while the base minor premises are the specific items of evidence of what the law states. The polysyllogistic approach thus serves as a formal proof of the pluralistic nature of legal reasoning.
Part V illustrates the limits of syllogistic reasoning by demonstrating how judicial reasoning in hard cases proceeds not by deducing conclusions from factual premises, but rather by evaluating the weight of competing arguments. Furthermore, rather than a "chain" of syllogisms, a more appropriate metaphor for legal reasoning is a "cable" of arguments that acquires persuasive force from the confluence of the different types of argument. Ultimately, the persuasiveness of a legal argument depends upon its susceptibility to attack within each category of legal argument and upon the relative weight accorded to the different categories of legal arguments in the context of the particular case.