Induction and Deduction

30.01.2018 |

Episode #6 of the course Logic basics: Understanding arguments by Gary Curtis


Welcome back! In today’s lesson, we’ll talk about the distinction between two types of argument: inductive and deductive.


Inductive vs. Deductive

All the way back in the second lesson, you learned that “arguments” are sets of statements, one of which is inferred from the rest. What I didn’t mention then is that there are two different ways a statement can be inferred from others. To be more exact, there are two levels of logical strength that can be claimed for the inference:

1. Necessity: It is claimed that the premises necessitate the truth of the conclusion, which is the strongest possible logical connection between statements. Such arguments are called “deductive.”

2. Probability: It is claimed that the premises make the conclusion more likely to be true than false. Clearly, this is a weaker claim than #1. When it is claimed that the conclusion of an argument follows from the premises with probability only, the argument is called “inductive.”

Notice that both inductive and deductive arguments are arguments that claim to have these levels of logical strength. However, sometimes the claim is wrong: For example, sometimes a deductive argument does not support its conclusion with necessity, but only with probability, or does not support the conclusion at all.

Warning: It is often claimed that a deductive argument is one that proceeds from the general to the particular, while an inductive one is the opposite. However, this is old-fashioned terminology that is no longer used by logicians. Many deductive arguments do lead from the general to the specific, but many do not. Similarly, many inductive arguments go from the specific to the general, but some do not, and this is not their defining characteristic.


• “[B]etween the time the doll was placed on the platform and the time the theft was discovered the dauphin [doll] could not have been stolen. It follows, simply and inevitably, that the dauphin [doll] must have been stolen outside that period.” (Source: Ellery Queen, “The Dauphin’s Doll,” italics in the original)

A tip-off to the fact that this is a deductive argument is the occurrence of the word “must” in the conclusion. “Must” is a “deduction indicator” that signals that an argument is deductive. Just as “it follows … that” signals that the conclusion is, “the dauphin [doll] must have been stolen outside that period,” “must” indicates that the relation between the premise and conclusion is deductive. Synonyms of “must,” such as “necessarily,” “has to,” “with necessity,” “cannot be false,” etc., are also sometimes used as deduction indicators. Moreover, if you think about it, you’ll see that the conclusion does indeed follow with necessity, so it’s plausibly deductive.

• “[S]ince Alfred was an Englishman who lived—and died—in England, the great likelihood was that the blackmailer was English, too, you see.” (Ellery Queen, “Money Talks”)

In this argument, the phrase “the great likelihood” in the conclusion is an induction indicator. Similar words and phrases, such as “probably,” also can act as induction indicators.


What to Do If There Are No Indicators

As is the case for many argument indicators, these words and phrases may serve other purposes than deduction or induction indicators, so don’t jump to conclusions. Also, these words or phrases must be attached to the conclusion to indicate a deduction/induction in a way similar to conclusion indicators. Just as argument indicators are not always present in an argument, deduction/induction indicators are frequently missing from deductive/inductive arguments.

So, how can you tell whether an argument is deductive or inductive if it lacks indicators? Use your logical sense of what the argument in context means. Also, you can test the argument by adding a deduction or induction indicator to the conclusion. Does the argument still make sense with the added indicator? If you are still in doubt, give the argument the benefit of the doubt and assume that it is inductive because induction is a weaker relation and thus, easier to achieve than deduction.

In tomorrow’s lesson, you’ll learn how to apply what you’ve learned about argument indicators to analyze the structure of arguments.


Recommended book

The Deduction Guide by Louise Blackwood


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