Use Logic to Win Arguments: A Primer for Lawyers

Arguments are the primary tools of our trade and we cannot use them effectively unless we understand and obey the rules of logic. Despite their import, most students do not have an opportunity to study the principles of logic in law school. I find this tragic and believe that logic should be a required course for all law students.

In the meantime, lawyers and law students must take it upon themselves to learn the principles of logic necessary to craft persuasive arguments. In this post, I offer a basic primer on three tools of logic that are particularly important in the practice of law: deductive reasoning, inductive reasoning by generalization, and inductive reasoning by analogy.


I highly recommend that all Lawyerist readers check out Judge Ruggero Aldisert’s excellent book Logic for Lawyers : A Guide to Clear Legal Thinking. If you don’t have the time or inclination to read an entire book on the subject, Judge Aldisert also co-authored the article Logic for Law Students: How to Think Like a Lawyer, which is great reading for lawyers and students alike. The following primer summarizes the key points in Judge Aldisert’s article.

Deductive Reasoning

In deductive reasoning, a conclusion is compelled by known facts. Deduction is often expressed in the form of a syllogism, in which a conclusion is inferred from two known premises. The most ubiquitous syllogism is:

All men are mortal.
Socrates is a man.
Therefore Socrates is mortal.

First you have the major premise, which is usually a broad and generally applicable truth; here, that all men are mortal. Then you have the minor premise, which is usually a more specific and narrowly applicable fact; here, that Socrates is a man. And the conclusion is true as a consequence of the premises; here, that Socrates is mortal.

The principle behind a syllogism is that what is true of the universal is also true of the specific. In deductive reasoning, you reason from the general to the particular, so it is essential that the general statement is a universal truth. Consider the following flawed syllogism:

Some men are tall.
Socrates is a man.
Therefore Socrates is tall.

The statement, “some men are tall” does not allow you to deduct that if Socrates is a man, then he must be tall. Unfortunately, legal writing is replete with flawed syllogisms. Don’t anchor your own arguments in flawed syllogisms. And use your knowledge of logic to expose the flawed syllogisms in the arguments of opposing counsel. Words to watch for include: some, certain, a, one, this, that, sometimes, many, occasionally, once or somewhere.

To underscore the importance of deductive reasoning in law, Judge Aldisert outlines syllogisms from several watershed Supreme Court opinions, including the following syllogism from Marbury v. Madison:

The Judicial Department’s province and duty is to say what the law is.
The Supreme Court is the Judicial Department.
The Supreme Court’s province and duty is to say what the law is.

Law students should identify syllogisms when reading cases and use syllogisms in their outlines and on exams. Note that you will often have to rearrange sentences to locate the syllogisms in cases or other texts. And sometimes, only part of the syllogism will actually be expressed. Writers often omit parts of a syllogism, such as when a premise is obvious. Informal syllogisms in which there is an unstated premise are known as enthymemes. Moreover, legal arguments often consist of several syllogisms which build on one another, known as polysyllogisms. Here is an example:

All men are mortal. Socrates is a man. Therefore Socrates is mortal.
All mortals die. Socrates is mortal. Therefore Socrates can die.
People who can die are not gods. Socrates can die. Therefore Socrates is not a god.

As lawyers, we should ensure that all deductive arguments in our briefs and memos are supported by sound syllogisms. The Judge offers the following generic model, used by prosecutors in criminal cases, as a starting point to create your own syllogisms:

[Doing something] [violates the law.]
[The defendant] [did something.]
[Therefore the defendant] [violated the law.]

Syllogisms are tools to help you evaluate and tighten your legal analysis. They are useful in outlining your arguments or deconstructing the arguments of others. But to be logically sound, your arguments do not need to be expressed through syllogisms. The truth of a premise may be so obvious that writing the premise would make your writing tedious, particularly given that we write for sophisticated audiences.

Inductive Reasoning by Generalization

When you cannot rely on universals or settled law to provide a major premise to compel your conclusion, you need to build your own major premise through inductive reasoning. Inductive reasoning by generalization uses several specific facts to create a theory that explains relationships between those facts and supports your conclusion. The Judge offers the following example:

Plato was a man and Plato was mortal.
Julius Caeser was a man and Julius Caeser was mortal.
George Washington was a man and George Washington was mortal.
John Marshall was a man and John Marshall was mortal.
Ronald Reagan is a man and Ronald Reagan is mortal.
Therefore, all men are mortal.

To use inductive reasoning successfully, you need to ensure that your supporting facts represent an appropriate sample size and are representative. With inductive reasoning, you can never be certain that your conclusion is true, but through your supporting facts, you should be able to establish that your conclusion is highly probable.

Inductive Reasoning by Analogy

Another form of inductive reasoning common in law is analogy, in which you make one-to-one comparisons and draw similarities between two different things. Rather than reasoning from the general to the specific (deductive reasoning) or from the specific to the general (generalizations), analogy requires reasoning from the specific to the specific.

Analogy is a common part of everyday life and legal practice. For instance, I am a lawyer and I find Lawyerist to be useful to my practice, so I assume other lawyers will find Lawyerist useful to their practice, as well. The Judge offers the following formula for an analogy:

A has characteristic Y.
B has characteristic Y.
A also has characteristic Z.
Because A and B both have characteristic Y, we conclude that B also shares characteristic Z.

To use analogy in law, the Judge suggests that you (1) establish similarities between two cases; (2) announce the rule of law embedded in the first case; and (3) apply the rule of law to the second case. Successful analogy depends on the relevancy of the comparison. It is therefore important to detail the similarities between the cases and to acknowledge their differences. You must establish that the relevant similarities outweigh the relevant differences and therefore the outcomes should be the same.


  1. Avatar Steve Bolton says:

    Excellent article!!

    Steve Bolton

  2. Avatar Blindguides says:

    Thanks for such a useful article. While considering these issues, it dawned on me that the IRAC method of answering a legal issue is a syllogism. The rule is the major premise. The application of the rule to our facts is the minor premise. And, the conclusion is the,…(wait for it)…conclusion. Voilà. Thanks again.

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