Deduction

Elements of Deductive Writing

In deductive writing, deduction requires two key ingredients: truth and logic. To demonstrate how important these things are, here are examples of deduction lacking truth and logic.

Example of Untruthful Deductive Reasoning

Take the following deductive syllogism.

Obviously, something has gone wrong with this line of deductive reasoning, but what? Examine each of the premises (the first two lines).

• The first premise, “TVs are electronic,” seems okay. This is true.

• The second premise that “cats are TVs” is clearly incorrect. The premise is false, so the conclusion is incorrect.

This deduction contains a logical fallacy. A logical fallacy occurs when a conclusion is arrived at through faulty reasoning. This deduction specifically contains an informal logical fallacy, meaning that some part of it lacks truth.

Example of Illogical Deductive Reasoning

Take the following deductive syllogism.

The issue with this deduction is different from the last. First, both premises (the first two lines) are true. Second, notice how the two premises share no elements in common (unlike the last example). The first premise discusses fish in the ocean, while the second discusses dogs on land. Herein lies the deductive flaw of this example.

Deduction requires that the premises share elements in common. Because this example has no shared elements between the premises, you cannot draw an accurate deductive conclusion from them.

Like the example before it, this example contains a logical fallacy. However, it specifically contains a formal logical fallacy, where the issue lies in the lack of logic and not the lack of truth. This kind of error is also called a non-sequitur.

Fig. 1 - A non-sequitur is a kind of failed deduction.

Deductive Writing Example

Deductions do not always appear as three-line syllogisms. They can also be strung together in a prose essay. Here is how you might see a deductive line of reasoning appear on a timed test or in a paper or journal.

Can you piece together the deduction in this essay paragraph? Here is how you might rewrite the deduction as a categorical syllogism.

In an essay, this piece of deductive reasoning would only be one part of a larger argument. This larger argument would be clear in a thesis statement like this one.

In such a context, the deduction about Briar would be one small point to help establish Webb’s radicalization. It would serve alongside many other lines of reasoning to support the thesis.

Fig. 2 - Deduction supports a thesis statement.

Induction vs. Deduction

Induction is the opposite process of deduction.

Unlike deduction, induction is not flawless when properly executed. However, it can still be useful for essay writing and logical argumentation. Here’s an example of induction.

Inductive reasoning is the kind that you will often see employed in research studies. These studies survey many different people with specific results. From those specific results, researchers draw a larger conclusion about whatever it is they are testing.

As stated, though, induction is by no means flawless. If you take only one example and try to apply it to everything, that is a logical fallacy called hasty generalization.

This is a hasty generalization because the sample size is very small: one, in this case.

Difference Between Inductive and Deductive Methods of Report Writing

Now that you understand the difference between deduction and induction, here is the difference between a deductive and inductive report.

This would be like an essay for a timed test for English class.

This would be like a scientific survey or report.

In most instances of persuasive writing, you want to write a deductive essay. This deductive essay might contain inductive conclusions. For instance, you might use a research trial as evidence. However, the overall trajectory of your essay will be deductive. It will contain a rich thesis statement and attempt to narrow a diverse set of facts into specific support to back your thesis.