In this short video lesson, part of the Legal Fundamentals Explained (LFE) series, Pedro Moniz Lopes (University of Lisbon) reflects on the role of logic in legal thinking and explores what logic can realistically offer to legal practice. He outlines the advantages of understanding the fundamentals of logic when applied to legal reasoning and discusses the complex relationship – marked by both attraction and resistance – between logic and law.
Below, you can find the full transcription corresponding to this video lesson*.
Pedro Moniz Lopes
Hi, my name is Pedro Munish Lopez. I’m a professor at the University of Lisbon, and I’m here to talk to you about what one can expect and get from logic in the law. This is a 10-minute class, and I’ll be very brief on such a heavy topic. Everything said here can and should be developed further in specific classes on each topic.
My purpose, to the extent that I wish to be understandable, is not to go deep into formal logic, but to briefly outline the advantages of knowing a little logic when applying it to legal reasoning.
I will start by saying that traditionally, there is a kind of relation of attraction and repulsion between logic and law. It is not uncommon for legal theorists to become enamoured with logical reasoning and even become logicians themselves. That’s the attraction. But at the same time, it is not uncommon to find lawyers who describe others as formalists or pedantic if they apply logical tools.
The repulsion here is well described in a well-known passage by Oliver Wendell Holmes, the legal scholar and Supreme Court Justice. In his book The Common Law, he said something like, “The life of the law has not been logic; it has been experience.” Law, he argued, could not be treated as if it contained only the axioms and corollaries of a book of mathematics.
There, you can see the tension between law and logic – this sense of fear of a kind of logical theocracy or rigid formalism, perceived as cold, overly constraining, non-adaptive, impersonal, and ultimately unfair in its application to law.
There’s a lot of literature on this, but I’d like to point to two papers. One is a well-known paper by Susan Haack titled Logic and the Law: Something, but Not All, published in Ratio Juris in 2007. The other is a reply by Argentinian legal philosopher Carlos Alchourrón and Eugenio Bulygin titled What Can One Expect from Logic in the Law? Not Everything, but More Than Something. These works reflect the ongoing debate about whether applying logical tools to legal reasoning is advantageous or not.
This tale of attraction and repulsion revolves around the fact that logic is abstract. It involves the extraction of formal data. It is, generally speaking – although there are many systems of logic – closed, fixed, and complete. It constrains; it is rationally impossible to refuse; it is formal, objective, rigid, impersonal.
By contrast, legal reasoning is typically seen as specific, particular, concrete, related to the case at hand, open to balancing alternatives, persuasive, rhetorical, material, personal, evaluative, and flexible. Many people sympathize more with this open way of reasoning, and logic can appear as the enemy.
However, there are misunderstandings about what logic is and what it is not. Logic is not equivalent to rationality or reasonableness. It is certainly not equivalent to rhetoric, and it is not psychological. Logic is, as Quine, the famous philosopher and logician, called it, the science of necessary inference. It is the study of what constitutes good and valid reasoning.
As applied to law – and here I borrow from Dutch legal logician Aarnout Boodtman’s account – logic concerns how far the conclusion of a legal argument is sufficiently supported by its legal premises in an anti-psychological manner. It does not deal with whether someone feels compelled or has a certain mental state or attitude. Logic deals with the structure of thought, not the process of thinking.
Within logic, there are many systems, but a main division is between formal and non-formal logic. Formal logic deals with the conditions under which a given statement can be considered a conclusion drawn from certain other statements. Non-formal logic concerns how conclusions are drawn in various scientific fields. It is more tolerant and not reducible to logical formulas.
Before proceeding, it is important to clarify the distinction between valid and sound arguments. In logic, we deal with valid arguments. For an argument to be formally valid, the truth or falsity of its premises is irrelevant. You don’t need to know the truth of the premises to logically reason. Premises may be false, and yet the argument can still be valid.
Sound arguments, on the other hand, are both valid and have true premises. These are more important for lawyers. While valid arguments relate to internal justification – deriving a decision from general legal rules – sound arguments combine internal with external justification. The latter involves deriving premises from external assumptions, which may or may not be logical. It has to do with the appropriate selection of premises under a given criterion of soundness.
When discussing law and logic, we should moderate our expectations to avoid disappointment. Logic will not help identify sources, interpret texts, qualify cases, or perform legal balancing in the broad sense. It may help in adopting interpretive theories, making analogies, distinguishing cases, filling legal gaps, and differentiating between rules and principles.
Logic will most certainly help clarify legal sentences, use and defeat legal arguments, identify legal positions, justify legal decisions, identify legal gaps, assess complete legal systems, use precedents, and analyze the consequences of norm derogations.
So logic in law is, on the one hand, less powerful than some enthusiasts claim – it cannot solve all legal problems. It has nothing to do with determining the specific meaning of a text. But it can be very helpful in spotting conflicts between applicable norms and in resolving which norm applies to a particular case. These are logical steps where logic is indeed useful.
To finish, logic is not limited to deduction. There are many logical moves. The best known is deduction, which in legal reasoning is associated with modus ponens. I don’t have time to explain it in detail, but basically, it’s a non-expanding logical move where from the rule “All A’s that are B are C,” and the case “A is B,” we deduce “A is C.” For example, “All men are mortal. Socrates is a man. Therefore, Socrates is mortal.”
There is also induction, which reverses the structure. You start with particular cases – A is B, A is also C – and you infer a general rule, such as “All B’s are C.” For instance, you observe three white tigers and infer that all tigers are white. This is important in legal reasoning, particularly for justifying the creation of legal principles from specific rules with similar consequences.
Finally, there’s abduction. Though some debate whether it’s truly a logical move, it’s crucial. Abduction has nothing to do with alien abductions. It was championed by Charles Peirce, an American philosopher. In abduction, you start with a result (A is C), and you posit the best explanation: that A is B under a rule where all Bs are C. This is fundamental to evidentiary reasoning, which often involves selecting the best explanation for an event – be it a crime or another legal outcome. Abduction allows for reasoning through competing hypotheses about causes.
These were just some examples of how we can reason more rigorously and correctly using logical tools. As I mentioned, each of these topics could be divided into many more specific subjects, each deserving a dedicated class.
I hope you enjoyed it. Thank you very much.
(*some minor grammatical and changes have been introduced in order to make the reading more fluid, but in no way altering the content or the format of each speaker’s interventions).
The LFE video series is powered by the EU Horizon Twinning project “Advancing Cooperation on The Foundations of Law – ALF” (project no. 101079177). This project is financed by the European Union.
