What legal reasoning is: kinds of reasoning, the legal syllogism and where it runs out

Reasoning is moving from things you accept (premises) to a conclusion. Lawyers do this in four main ways, often within a single paragraph, and a good lawyer knows which one is being used because each can go wrong in its own way.

Deduction dominates the written form of legal reasoning, because a judgment must show how the result follows from a rule. Analogy and induction dominate the way rules are found and shaped, because most of the law that matters in a hard case is not stated in any rule. Inference to the best explanation dominates the way facts are found. Modules 2 to 7 take each in turn.

The basic shape of a legal conclusion is a syllogism: a rule (major premise), the facts (minor premise) and the result (conclusion). It is deduction, and its strength is that anyone can check each part.

A conclusion can be wrong in three ways, and each points to a different kind of work.

• The rule is wrong or incomplete. There may be an exception, a later amendment or a different statute. This is a research problem.
• The facts are wrong or unproven. Perhaps Mr Aldous did give a note. This is a problem of evidence (Module 6).
• The rule does not fit the facts. Is an email ‘written notice’? Is a note left under the door ‘given’? This is a problem of interpretation (Module 2), and it is where most legal arguments happen.

In an easy case, all three are settled and the syllogism does the whole job. In a hard case, at least one of them is open, and the lawyer’s task is to argue for the version that favours the client, using reasons that a court could accept.

If law were only syllogisms, courts would be calculators. They are not, because rules are written in language, by people who cannot foresee everything. The legal philosopher H. L. A. Hart described this as the ‘open texture’ of law: every rule has a core of clear cases and a penumbra of doubtful ones. His famous illustration is a rule that says ‘No vehicles in the park’. A car is plainly covered. An ambulance on an emergency call, a child’s tricycle, a war memorial that is a decommissioned tank, an electric scooter: are they vehicles? The words do not decide it.

At these points the lawyer must give reasons that go beyond the words: the purpose of the rule, how earlier cases have treated similar facts, the consequences of each reading, and the principles of the legal system. That is what Modules 2 to 4 and 7 are about. The important point for now is that this is not a failure of law. It is how the law works, and it is why argument matters.

Law uses two broad forms of directive, and they invite different reasoning.

Law mixes the two. A tax statute is mostly rules. Negligence is a standard (‘reasonable care’). Many legal systems also gradually convert standards into rules: the first cases apply ‘reasonable care’ case by case; later courts say that a driver who passes a stopped school bus has not exercised it, as a matter of law. When you read an authority, ask whether it is giving you a rule or a standard, because the kind of argument that fits is different.

A distinguishing feature of legal decision-making is that results must be justified by reasons that are open to inspection. A judge who simply announced ‘the claim fails’ would not be doing law. The requirement has several purposes.

• Fairness to the parties. The loser is entitled to know why.
• Control. A higher court can only review what the lower court explained.
• Guidance. Reasons turn one decision into a rule for later cases.
• Discipline. A decision-maker who must write down a reason cannot rely on prejudice or whim without it showing.

A distinction worth drawing is between an explanation of why a decision was made (the judge was tired and disliked the claimant) and a justification of it (the statute requires this result). Legal reasoning is concerned with justification: reasons that a neutral observer could accept as good ones, even if they disagree with the result.

Arguments can be assessed on several dimensions, and a good legal argument does well on all of them.

An argument can be valid but unsound (it reasons well from a false premise), or sound but unpersuasive (the law is on your side but the result looks unjust, and the court looks for a way round). The best arguments are those in which the law, the facts and the sense of justice all point the same way, or those that explain why a surprising result is nevertheless right.

The rest of the course gives you tools to test each of these: the Precedent Comparer for analogy, the Argument Builder for structure and the Fallacy Spotter for weak moves.