Inductive and Deductive Reasoning (Induction vs. Deduction)

EVEN IN GRADE SCHOOL, we were taught that in communication, we use words, sentences, and paragraphs. Words compose a sentence and sentences in turn compose a paragraph. You would notice that many of our paragraphs contain reasoning that is in the form of arguments.

Argument basically is a group of statements, one of which is the conclusion (what is being proved) and the rest are the premises (the bases for the conclusion). We commonly use and encounter two kinds of argument based on the two kinds of reasoning that we make: deductive and inductive. These two basic forms of reasoning are as well recurrently employed by debaters. So what’s the difference between induction and deduction?

Inductive reasoning

Induction or reasoning inductively is basically inferring a general conclusion from a collection of particular facts. For example, one might conclude that “All flowers are fragrant” because orchid, dahlia, rose, and santan are fragrant.

Induction is also inferring or reaching a conclusion based on observations. For instance, after witnessing for years that grasshoppers invade our rice plants during summer, we may conclude that next summer our rice plants will again be invaded by grasshoppers. The observations or assumptions on which we draw conclusions, say the annual invasion of grasshoppers, constitute the premises or assumptions of inductive arguments.

In induction, the support provided by the premises for the conclusion can vary in strength depending on how likely it is that the conclusion will be true, assuming all of the premises are true. Thus, inductive argument is either reliable (strong) or unreliable (weak).

One way to create a reliable inductive inference is to base its conclusion on ample amount of individual representative instances. Let us examine this argument:

According to a certain survey, 385 of the 500 respondents say they would like Mr. Crisostomo to be the next mayor in our town. Therefore, the majority of our voters would vote for him if he decides to run for the position.

In this argument, whether or not the evidence is adequate depends. In a town of 5,000, the 500 citizens are a ten percent sample, perhaps enough for the purposes of survey for election. But in a town of 60,000 residents, the respondents would amount to less than one percent of the total number of residents—an insufficient sample on which to base a conclusion regarding the possible election results.

Whether or not the evidence is representative again depends. We may believe that the survey presents representative sample if we knew that it was thoroughly constructed so as to reflect the sex, age, race, civil status, job, religion, and income of the town’s populace as a whole.

Deductive reasoning

In Logic, deduction is a process of reasoning in which reasons are given in support of a claim. An argument is thus deductive if the premises claim to give conclusive grounds for the truth of the conclusion. In other words, the premises claim to support the conclusion with necessity. Hence, deductive argument is either valid or invalid. In a valid deductive argument, the conclusion follows necessarily from the premises. Consider this argument:

All dogs are mammals.
Canines are dogs.
Therefore, canines are mammals.

This deductive argument is valid. If we assume the premises “All dogs are mammals” and “Canines are dogs” to be true, then we are necessitated to accept the conclusion “Canines are mammals” as also true. It is inconsistent and self-contradictory to accept its premises but deny its conclusion, because the conclusion necessarily follows from the premises.

Remember, however, that the actual truth or falsity of the premises and the conclusion is not at issue in determining whether an argument is a valid deduction. Let’s take this example:

Reptiles have mammary glands.
Lizards are reptiles.
Therefore, lizards have mammary glands.

The first premise in this argument is in fact false, and so is the conclusion. But the argument is still deductively valid because its form adheres to the minimum requirement of a valid deductive argument: If the premises were true, the conclusion would have to be also true.

In contrast, in an invalid deductive argument, the conclusion does not follow necessarily from the premise or premises. Consider this argument:

Paola loves good food; therefore, she will be an excellent chef.

This argument is invalid because the conclusion does not follow necessarily from the premise. Even if we accept the premise “Paola loves good food” as true, the conclusion “She will be an excellent chef” remains to be either true or false. Meaning, there will be no inconsistency to consider the conclusion as false even if we take the premise as true. Paola’s love of good food does not guarantee her supposed ability to cook well.

Centrally deductive, Logic as an academic subject focuses on various forms of valid argument. In fact, seldom do we find a Logic syllabus (and a Logic textbook, for that matter) that focuses on inductive reasoning. In the subject Debate on the other hand, inductive arguments are as usable as the deductive ones. 

Induction vs. deduction

Induction and deduction are somehow similar in the sense that both give much emphasis on the likelihood of the conclusion’s being true if the premises were true, that is, the support that the premises provide for the conclusion. Technically however, their resemblance ends there.

We have learned that in a valid deductive inference, the premises support the conclusion in such a way that it would be impossible for the premises of an argument to be true and for its conclusion to be false. This spells its sharp contrast to induction, for the truth of the premises of an inductive argument does not guarantee the truth of its conclusion. Meaning, even when an argument is inductively strong, the possibility remains that its conclusion false even if its premises are true.

Going back to our example, even though it is true that grasshoppers have invaded our rice plants for years, it remains possible that grasshoppers would not destroy our farm this summer, or never reappear at all. From an inductive point of view though, to infer that the grasshoppers will return is still correct. Technically therefore, inductive inferences, though not worthless, are deductively invalid.

Application

Deduction and induction are both used in everyday conversations and even in scientific reasoning. Bringing together these two forms of reasoning are effective in establishing general laws, drawing conclusions about a population, predicting the occurrence of a future event based observations of similar past events, and drawing conclusions about causes of an illness based on observations of symptoms. Thus, utilizing both of them in a debate is advisable and even ingenious.

Just a word of caution: Do not confuse inductive argument with invalid deductive argument. The difference between deduction and induction is not the difference between good and bad reasoning, but between two ways to support the truth of conclusions. In creating argumentative paragraph in writing or speaking (like in Debate), Logic prescribes that we can use both valid deductive and reliable inductive argument. In summary, what Logic warns us from when it comes to creating arguments is the use of invalid deductive and unreliable inductive argument. (© 2014 by Jensen DG. Mañebog/MyInfoBasket.com)

ALSO CHECK OUT:
Reasoning and Debate: A Handbook and a Textbook by Jensen DG. Mañebog

Also Check Out: From Socrates to Mill: An Analysis of Prominent Ethical Theories, also by author Jensen DG. Mañebog

INTERACTIVE ONLINE ACTIVITY

Among the articles and comments in OurHappySchool.com site, look for five (5) strong inductive and five (5) valid deductive arguments. Print screen them to a long bond paper. For each argument, write a two-sentence explanation: state the title of the article from which it was taken and justify the argument’s validity or strength. Submit the output to your professor.

SUPPLEMENTARY ONLINE READING

Look for the article “A Short History of Logic” through the search engine (upper right section) of www.OurHappySchool.com. The lecture teaches, among other things, how the formal study of correct reasoning developed throughout history.