In terms of logic – Part one

Naturally, lawyers often use terms of logic in discussing their arguments and those of the other side. Sometimes they don’t use the terms quite right. Here’s a short list of commonly-used terms and how to use them correctly. More next week.

Let’s first lay out a simple logical argument to use as an example throughout.

Premise 1: Jill punched Jack.

Premise 2: Punching someone is assault.

Conclusion: Jill has committed an assault.

A premise is a fact upon which a conclusion depends. If your argument is sound (see below), you should be very explicit in your briefing about what the premises are and what your conclusion is. A clearly-described sound argument is very hard to defeat. If your opponent’s argument is not sound, it is an equally good idea to lay it out clearly in your briefing, so that its flaws will show.

Three terms of logic that are (incorrectly) used interchangeably by many lawyers are true, sound, and valid.

True. We all understand true. A factual claim is true if it accurately describes reality, and untrue otherwise. Only a factual claim may be true or untrue; truth is not a property of arguments. In the example, both premises are factual claims (the first one about an event in the world, the second about a question of law).

Valid. Validity is a property of arguments, not of factual claims. An argument is valid if the conclusion logically follows from its premises. The Jill and Jack example above is a valid argument.

Sound. Soundness is also a property of arguments. If an argument is valid and its premises are true, the argument is sound. An argument is unsound if it relies on a premise that is not true or if it is invalid.

You may well be understood even if you misuse logical terms (saying that a factual claim is sound when you mean that it is true, for example), but you will annoy any judge who knows better, and in particular any who was trained in logic or rhetoric. Logic and rhetoric used to be part of any fundamental education, and still are taught under many different rubrics. You should assume that your judge knows them.

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