Inductive and deductive reasoning. Traditionally, logical arguments are classified as using either inductive or deductive reasoning; in practice, the two almost always work together. Inductive reasoning is the process of making a generalization based on a number of specific instances. If you find you are ill on ten occasions after eating seafood, for example, you will likely draw the inductive generalization that seafood makes you ill. It may not be an absolute certainty that seafood is to blame, but the probability lies in that direction.
Deductive reasoning, on the other hand, reaches a conclusion by assuming a general principle (known as a major premise) and then applying that principle to a specific case (the minor premise). In practice, this general principle is usually derived from induction. The inductive generalization Seafood makes me ill, for instance, could serve as the major premise for the deductive argument Since all seafood makes me ill, the shrimp on this buffet is certain to make me ill.
Deductive arguments have traditionally been analyzed as syllogisms, reasoning that contains a major premise, a minor premise, and a conclusion.
| major premise | All people die. |
| minor premise | I am a person. |
| conclusion | I will die. |
Syllogisms, however, are too rigid and absolute to serve in arguments about questions that have no absolute answers, and they often lack any appeal to an audience. Aristotle’s simpler alternative, the enthymeme, asks the audience to supply the implied major premise. Consider the following example:
Since violent video games can be addictive and cause psychological harm, players and their parents must carefully evaluate such games and monitor their use.
You can analyze this enthymeme by restating it in the form of two premises and a conclusion.
| major premise | Games that cause harm to players should beevaluated and monitored. |
| minor premise | Violent video games can cause psychological harm to players. |
| conclusion | These games should be evaluated and monitored. |
Note that the major premise is one the writer can count on an audience agreeing with or supplying: safety and common sense demand that potentially harmful games be used with great care. By implicitly asking an audience to supply this premise to an argument, a writer engages the audience’s participation.
Toulmin’s system (10d) looks for claims, reasons, and assumptions instead of major and minor premises.
| claim | Parents should not allow children to play violent video games. |
| reason | Exposure to violent video games may make children more indifferent to violence. |
| assumption | Parents do not want their children to be indifferent to violence. |
Whether it is expressed as a syllogism, an enthymeme, or a claim, a deductive conclusion is only as strong as the premise or reasons on which it is based.