Inductive vs. Deductive Reasoning

The Logical Structure of Arguments

By Professor Tammy Peery

As you plan an argument, itís important to consider the direction in which you plan to set up your proof.  Induction and deduction are two methods of structuring logical arguments to consider as you plan your essay.  They work in essentially opposite directions.

INDUCTION moves from specific observations to general conclusions, like a scientific lab experiment.  Based upon lots of specific examples, you are able to draw a general conclusion.    A way to visualize induction is a triangle with a narrow point (one specific starting example) at the top that gets larger as it moves toward the base (a general conclusion built by many specific examples):

For example, think of completing these statements:

If I make sure to get eight hours of sleep tonight, then  __________.

If I eat like a horse every day this week, then __________.

If I donít study at all for an exam, then  ________________.

Every day you observe and experience many things. You might conclude the last statement by saying, ďIf I donít study at all for an exam, then I will fail it.Ē This conclusion is probably based upon many individual experiences: you donít study for history, you fail the exam; you donít study for math, you fail the exam; you donít study for science, you fail the exam; you donít study for English, and you fail yet another exam. Based upon these specific observations, you then draw the general conclusion that if you donít study for any type of exam, you will fail it. You have just used induction.  In other words, if you plan to use lots of specific examples to reach your general argument conclusion, then youíll plan an inductive essay.

An example that uses a primarily inductive strategy is taken from ďBad to the Last DropĒ by Tom Standage. In order to prove the general conclusion that bottled water is not worth drinking, he provides many specific, individual examples.  He starts with an inductive experiment:  blindly taste several different types of water to see what tastes best.  If the first bottled water tastes worse than tap, and the second bottled also tastes worse than tap, etc, we make the inductive leap that all bottled waters probably taste worse than tap.  As his essay continues, he provides many specific examples and statistics (cost, taste, nutritional benefits, etc) to support his general conclusion that tap water is a better choice to make than bottled water.

Unlike some other forms of logic, induction does not work 100% of the time. This is because to draw a conclusion, you must make an inference, or inductive leap. An inductive leap draws a conclusion about the unknown based on the known. In the above example, you canít be absolutely certain that after failing all those other exams, if you donít study for  your art exam then you will fail it as well.  Failing the art exam is certainly probable, but not a sure thing.  Similarly, unless you taste all of the bottled waters in existence, you canít be 100% certain that they all taste worse than tap.  Three factors to consider are that your sample (the observations on which you base your argument) must be sufficient (you must make enough observations Ė taste 100 bottles of water not 3), representative (your observations must relate to the conclusion you hope to draw Ė you canít look at the shape of the bottle as part of your judgment, nor should you only look at generic brands), and known (it must be clear from what sources youíve drawn your observations Ė who provided those water bottles for your taste test?  Were they fresh? Chilled?). Naturally, the more observations we make, the narrower the gap between our observations and our conclusion, and the better our chance of drawing an accurate conclusion.  Even so, absolute certainty is not possible.  At some point, writers must decide they have enough evidence to present a convincing conclusion to their readers. 

DEDUCTION, oppositely, moves from a general observation to a specific conclusion, and -- when done correctly -- it works 100% of the time.  The trick is that itís very hard to do correctly! Most legal or courtroom arguments are based upon deductive reasoning since weíve set general rules that we agree to abide, then that general rule is applied to a specific case.  Deduction can be visualized as an inverted triangle; you begin with the broadest idea at the top and work your way to a more precise conclusion:

The process of deduction is called a syllogism.  A syllogism is a three part set of propositions or statements that contains a major premise, a minor premise, and a conclusion.

  • ?    The major premise makes a general statement that the writer believes to be true:

All books from that store are new.

  • ?    The minor premise presents a specific instance of the belief stated in the major premise:

These books in my hand are from that store.

  • ?    If the reasoning is sound, then the conclusion should follow from the premises:

Therefore, these books in my hand are new.

The strength of a deductive argument is that if readers accept the premises, they usually grant the conclusion.  You do, however, need to make sure that each of your premises is both true (factually accurate) and valid (structured correctly).  You can create bad syllogisms if you arenít careful; for example:?

            Major premise:          All bottled water tastes bad.

            Minor Premise:         Dasani is a type of bottled water.

            Conclusion:              Therefore, Dasani tastes bad.?

Because this major premise is not true, the syllogism is entirely false.  There are many people who would disagree with the idea that all bottled water tastes bad, so the major premise is false. If any portion of the syllogism is not factually accurate, then the syllogism isnít true, and it falls apart. 

In addition to being factually accurate, a syllogism must be structured properly, or valid, to work:

            Major premise:  the Coca Cola company makes bottled water.

            Minor Premise:  Evian is a type of bottled water

            Conclusion:      Therefore, Evian is produced by the Coca Cola Company

Even though the premises of this one are true, the conclusion does not follow logically This is an example of an invalid syllogism.  This is because the middle term must be the one that comes at the beginning of the the major premise. ?

            Major Premise:          All Dasani water is produced by the Coca Cola company

            Minor Premise:         This is a bottle of Dasani water.

            Conclusion:              This bottle of water is produced by the Coca Cola company

This version is logical: a syllogism that is both true and valid.  In other words, it is both factually accurate and correctly structured.

Also beware of enthymemes.  These are syllogisms where the major premise is missing, usually in order to mislead the reader.  For example, the statement ďDonít pay any attention to Father Smithís opinion on abortion Ė heís Catholic and a man--  is based upon the unstated (false) premise that no Catholics or men can have a valid opinion on abortion. (example from MacDonald & Burtonís The Language of Argument)

Most of the time we learn through induction, then apply what weíve learned to other things through deduction.  For example, babies are the ultimate inductive thinkers.  They try things again and again and again until they figure out how something works.?

I put my finger in the outlet in the living room; I get zapped.  I put my finger in the outlet in the dining room; I get zapped.  I put my finger in the outlet in the kitchen I get zapped.  Eventually, baby learns from experience that every time he puts his finger in an outlet he gets zapped (he makes an inductive leap based upon a number of specific observations to get to this general conclusion) 

Once heís made that leap, he is able to think deductively about the situation:?

All outlets zap when touched

Aunt Kellyís house has outlets just like mine

The outlets at Aunt Kellyís house will zap me just like the ones at home (so I wonít touch!)

The main goal of using induction or deduction for ENGL102 is to set up a clear structure that your essay can follow.  Do you want to start off small & provide lots of specific examples to prove that something universally happens?  Do you want to start off with a universally accepted idea and show that your particular point is an example of that idea?  Many more complicated logical structures can be developed further with these strategies as well, as you may learn if you take a philosophy of logic class.  However, the basic structure is what is important for this class.