The Use and Limits of Deductive Logic in Legal Reasoning

Abstract:     
At one time law was considered to be a science; this belief was associated with the concept of “natural law.” And just as law was considered a science, legal reasoning was considered to be a species of deductive logic. However, it is now recognized that the purpose of legal reasoning is not to prove to others the truth of a statement of fact, but is rather to persuade others about how the law ought to be interpreted and applied. Although legal reasoning is logical in form, in substance it is evaluative.

There are two types of hard cases: cases where a rule of law is ambiguous, and cases where the validity of a rule is in question. Questions of ambiguity arise when the minor premise of a proposition of law is challenged, while questions of validity arise when the major premise of a proposition of law is challenged. Hard cases are cases where two or more valid legal arguments lead to contradictory conclusions.

The brief of a case is not a single syllogism of deductive logic; rather, it consists of strands or chains of syllogisms - "polysyllogisms." The polysyllogistic approach is a useful means for describing the underlying structure of legal reasoning. This approach reveals that the base minor premises of legal arguments consist of items of evidence of what the law is, while the base major premises are the categories of legal arguments that may be legitimately made.

Deductive logic plays a central role in legal reasoning, but logic alone cannot solve hard cases. When we attempt to reduce the decision of a case to an argument of deductive logic, the aspects of legal reasoning that are not deductive are exposed. A system of pure logic works only in easy cases, i.e. cases where the validity of the rule of law is unchallenged and the terms of rule are unambiguous. Hard cases are resolved by a complex balancing of intramodal and intermodal arguments, in which the court evaluates not only the strength of individual arguments, but also the relative weight of the values that support our legal system, as implicated in the particular case.

Number of Pages in PDF File: 50

Keywords: logic, deductive logic, syllogism, syllogistic reasoning, legal reasoning

JEL Classification: K10, K19

Accepted Paper Series