Logics of argumentation and the law

Introduction In his LEGAL TRADITIONS OF THE WORLD (Glenn 2010), Patrick Glenn observes that the world contains many different legal traditions, often inconsistent with each other, and that even a single tradition can contain different sub-traditions that may be inconsistent with each other. Moreover, he notes that these traditions may interact with each other in complex ways. In chapter 10, Glenn raises the question of how to account for this from the perspective of formal logic. In chapter 14 of his THE COSMOPOLITAN STATE (Glenn 2013) he writes that new logics may be needed that are multivalent, paraconsistent or non-monotonic and do not adhere to the classic rules of non-contradiction and the excluded middle. In this chapter I will explore the use of one such new kind of logic, namely, logics of argumentation. It will turn out that such logics offer what Patrick Glenn is asking for without giving up classical two-valued logic. Instead, classical logic is in argumentation logics embedded in a larger formal framework, and it is this larger framework that has the desired nonstandard behavior. Thus argumentation logics provide a way to cope with inconsistent legal traditions without having to give up two-valued logic, a way that is moreover arguably close to the way lawyers think since notions like argument, counterargument and rebuttal are natural them. Introductory textbooks to logic often portray logically valid inference as ‘foolproof’ reasoning: an argument is deductively valid if the truth of its premises guarantees the truth of its conclusion. In other words, if one accepts all premises of a deductively valid argument, then one also has to accept its conclusion, no matter what. However, we all construct arguments from time to time that are not foolproof in this sense but that merely make their conclusion plausible when their premises are true. For example, if we are told that John and Mary are married and that John lives in Amsterdam, we conclude that Mary will live in Amsterdam as well, since we know that usually married people live where their spouses live. Sometimes such arguments are overturned by counterarguments. For example, if we are told that Mary lives in Rome to work at the foreign offices of her company for two years, we have to retract our previous conclusion that she lives in Amsterdam.