Why Is Legal Reasoning Defeasible ? ∗ Juan

This paper analyses the claim that legal reasoning is defeasible, which is indeed a hallmark of some major contributions to the theory of legal reasoning in recent times. Before addressing the question of what kind of logical tools are needed to formalize defeasible reasoning, it must be explained why legal reasoning is supposed to be defeasible in the first place. Some arguments to this effect are taken into account (having to do with the allocation of burdens of proof in legal procedures, reasoning with incomplete information and the proper way of individuating norms), but it is held that none of them really proves that legal reasoning cannot be reconstructed as a deductive inference. The strongest argument to justify the claim that legal reasoning is defeasible seems then to be that all legal norms turn out to be defeasible: but here this argument is disputed, trying to show that it would lead us to embrace either wholesale indeterminacy or ‘legal particularism’, which is criticised as an untenable form of conceiving legal justification. Finally, it is suggested that there is indeed some limited sense in which it could be said that legal reasoning is defeasible (having to do with the idea that justification in law is a matter of coherence), but it is not grounded on the possibility that legal norms themselves be defeasible as well, and especially it does not call in question the subsumptive character of legal justification. 1. Legal theory behind a theory of legal reasoning Some major contributions to the theory of legal reasoning appeared in recent times assume as a seemingly indisputable starting point that it is defeasible. Of course, the claim that defeasibility is an essential trait of legal reasoning is by no means novel. But only recently has this claim been put forward in a thorough and systematic way, making use of a set of notions and tools developed in the field of artificial intelligence. As it is upheld nowadays, the claim seems really to be twofold. First, on what could be called its jurisprudential side, the contention is that legal reasoning does have some distinctive features that make it non-monotonic. Therefore, any version of the claim includes some explanation about what is supposed to be the real source of defeasibility in legal reasoning. And secondly, on its logical side, what is contended is that classical logic is not suitable to formalize legal reasoning precisely because it is defeasible. Then, taking for granted that we require new logical tools to cope with defeasibility, the claim is coupled in its different versions with several proposals as to how the allegedly needed non-standard logic should look like. Nevertheless, the claim has been contested on both sides. As for the logical one, there are indeed some nagging doubts about the very idea of working out nonmonotonic logics. It is sometimes said that it makes no sense to speak of a nonmonotonic ‘logic’ unless one is prepared to accept a non-monotonic notion of inference. However, the idea of a defeasible conditional as a new kind of logical connective is often deemed to be flawed, indeed the product of conflating a standard notion of material implication with the change of our premises in a process of rational belief revision. Hence, this purportedly new logical connective would hardly have any practical use, leading to systems of so-called ‘non-monotonic logic’ computationally inefficient. From this point of view there is no need to deny the genuine interest of framing models that intend to give formal expression to patterns of rational belief revision in non-monotonic or default reasoning: what is stressed is that it would be better not to think of them as ‘logics’ properly said. Of course, to this it can be replied that all hinges on what ‘a logic properly said’ is supposed to be. These are complicated ∗ This essay has been published in A. Soeteman (ed. by) Pluralism and Law, Kluwer Academic Publishers, 2001