## Reason Based Logic in Law

### H. Bart Verheij

#### Abstract

Reason based logic aims to reflect the nonmonotonic character of human reasoning. More specifically, it is inspired by logical problems that arise in the field of law. It essentially differs from other approaches such as circumscription, default logic and autoepistemic logic in that it allows the weighing of reasons.

Reason based logic distinguishes between propositions and rules. A rule is not treated traditionally as a material implication, but as an object that may constitute a reason. To derive a conclusion, reasons for and against it are collected and weighed. Whichever group of reasons wins, if any, determines whether the conclusion, its opposite or neither one can be derived.

Reasons arise in reason based logic through the application of rules. A rule consists of a condition and a conclusion. If the rule is applicable, the condition of a rule is considered to be a reason for its conclusion. The applicability of a rule must itself be derived on the basis of reasons. An important reason pleading for the applicability of a rule is that its condition is satisfied. Reasons against the applicability of a rule are for example that application is against the rule's purpose, or is in conflict with a more specific, more recent, or superior rule.

The weighing of reasons is based on the following principle: if all reasons point in one direction, i.e., all the reasons plead for a statement or all against, the statement or its negation can be derived. In other cases no decision follows, unless extra information is available. Such information can for instance be derived from previous cases.

Reason based logic is nonmonotonic in the sense that by the addition of facts or rules conclusions can become underivable. This can have two causes. First, new information can give rise to new reasons and thus change the outcome of weighing. Second, new information can make rules inapplicable and therefore invalidate reasons.

The paper offers a formal description of reason based logic and demonstrates its use by means of classical examples of nonmonotonic logic. Amongst others it is illustrated how reason based logic deals with the Nixon diamond and the Yale shooting problem. We also discuss an implementation in Prolog of an inference mechanism based on reason based logic. Several examples show how the ideas can be applied in the field of law.

Reference:
Verheij, H. Bart (1995). Reason Based Logic in Law. Verso un sistema esperto giuridico integrale (eds. C. Ciampi, F. Socci Natali and G. Taddei Elmi), pp. 681-693. CEDAM, Padova. Also published as report SKBS/B3.A/93-18.