Logos: the appeal to reason
Logos: inductive reasoning

Logos: deductive reasoning (syllogism)

In the last few days, I’ve been posting on classical rhetoric and its lessons on persuasion. I’ve mentioned the three modes of persuasion, including logos, the appeal to logic or reason. And last Friday, I mentioned three aspects of logos: deductive reasoning, inductive reasoning, and reasoning by analogy. Today we’re going to dig more deeply into deductive reasoning.

You’ve probably been using deductive reasoning since law school without knowing its name. As Judge Aldisert observed, “Deductive reasoning is a mental operation that a student, lawyer or judge must employ every working day.” Ruggero J. Aldisert, Logic for Lawyers 45 (3d ed. 1997). In their book Making Your Case, Justice Scalia and Bryan Garner said, “If you have never studied logic, you may be surprised to learn—like the man who was astounded to discover that he had been speaking prose all his life—that you have been using syllogistic reasoning [i.e. deductive reasoning] all along.” Antonin Scalia and Bryan A. Garner, Making Your Case 41 (2008).

In deductive reasoning, we deduce the truth of a statement (the conclusion) from the truth of two other statements (the premises). The classic means of articulating deductive reasoning is called the syllogism. A syllogism usually consists of a major premise, a minor premise, and a conclusion that must follow from the premises. Often, the major premise makes a categorical assertion, and the minor premises puts someone or something in that category, so that what is true of the category must be true of that someone or something. The classic example of a syllogism concerns the mortality of the philosopher Socrates:

Major premise: All men are mortal.
Minor premise: Socrates is a man.
Conclusion: Therefore, Socrates is mortal.

When a syllogism is validly constructed and its premises are true, then its conclusion is inevitably true. This makes the syllogism a powerful persuasive tool. This is why brief-writing experts recommend using syllogisms in crafting issue statements and point headings for briefs. See Making Your Case at 41–43, 83–88, and 100; Bryan A. Garner, The Winning Brief 80, 93, 111–20, and 407–22 (3d ed. 2014).

When your opponent argues in the form of a syllogism, you have two ways to attack the argument: (1)  Show that one or both of the premises are false. (2) Show that the syllogism is invalidly constructed, i.e., that it contains a logical fallacy. To do the latter, you need to know your fallacies. You can find discussions of logical fallacies in Judge Aldisert’s book and in Corbett and Connors’s textbook. Those books and other recommended readings are listed below:

  • Ruggero J. Aldisert, Logic for Lawyers 45–46 (3d ed. 1997) (on deductive reasoning generally); id. at 145–68 (on formal fallacies).
  • Tessa L. Dysart, Leslie H. Southwick, and Ruggero J. Aldisert, Winning on Appeal § 11.4 (3d ed. 2017).
  • Edward P.J. Corbett and Robert J. Connors, Classical Rhetoric for the Modern Student 38, 42–60 (4th ed. 1999) (on deductive reasoning and syllogism), id. at 65–71 (on fallacies).
  • Antonin Scalia and Bryan A. Garner, Making Your Case 41–43, 83–88, and 100 (2008) (using syllogistic reasoning in brief writing).
  • Bryan A. Garner, The Winning Brief, Tips 14–16 (3d ed. 2014).

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