Deductive and Inductive Arguments

In this tutorial you will learn to distinguish deductive arguments from inductive

arguments.

Chapter 3.a

All bats are mammals.All mammals are warm-blooded.So, all bats are warm-blooded.

All arguments are deductive or inductive.

Deductive arguments are arguments in which the conclusion is claimed or intended to follow necessarily from the premises.

Inductive arguments are arguments in which theconclusion is claimed or intended to follow probably from the premises.

Is the argument above deductive or inductive?

All bats are mammals.All mammals are warm-blooded.So, all bats are warm-blooded.

Deductive.

If the premises are true, the conclusion, logically, must also

be true.

There are four tests that can be used to determine whether an argument is deductive or inductive:

•the indicator word test

•the strict necessity test

•the common pattern test

•the principle of charity test

Kristin is a law student.Most law students own laptops.So, probably Kristin owns a laptop.

The indicator word test asks whether there are any indicator words that provide clues whether a deductive or inductive argument is being offered.

Common deduction indicator words include words or phrases like necessarily, logically, it must be the case that, and this proves that.

Common induction indicator words include words or phrases like probably, likely, it is plausible to suppose that, it is reasonable to think that, and it's a good bet that.

In the example above, the word probably shows that the argument is inductive.

No Texans are architects.No architects are Democrats.So, no Texans are Democrats.

The strict necessity test asks whether the conclusion follows from the premises with strict logical necessity. If it does, then the argument is deductive.

In this example, the conclusion does follow from the premises with strict logical necessity. Although the premises are both false, the conclusion does follow logically from the premises, because if the premises were true, then the conclusion would be true as well.

Either Kurt voted in the last election, or he didn't.Only citizens can vote.Kurt is not, and has never been, a citizen.So, Kurt didn't vote in the last election.

The common pattern test asks whether the argument exhibits a pattern of reasoning that is characteristically deductive or inductive.

If the argument exhibits a pattern of reasoning that is characteristically deductive, then the argument is probably deductive.

If the argument exhibits a pattern of reasoning that is characteristically inductive, then the argument is probably inductive.

In the example above, the argument exhibits a pattern of reasoning called "argument by elimination.“

Arguments by elimination are arguments that seek to logically rule out various possibilities until only a single possibility remains. Arguments of this type are always deductive.

Arnie: Harry told me his grandmother recently climbed Mt. Everest.Sam: Well, Harry must be pulling your leg. Harry's grandmother is over 90 years old and walks with a cane.

In this passage, there are no clear indications whether Sam's argument should be regarded as deductive or inductive. For arguments like these, we fall back on the principle of charity test.

According to the principle of charity test, we should always interpret an unclear argument or passage as generously as possible.

We could interpret Sam's argument as deductive. But this would be uncharitable, since the conclusion clearly doesn't follow from the premises with strict logical necessity. (It is logically possible--although highly unlikely--that a 90-year-old woman who walks with a cane could climb Mt. Everest.) Thus, the principle of charity test tells us to treat the argument as deductive.

Tess: Are there any good Italian restaurants in town?

Don: Yeah, Luigi's is pretty good. I've had their Neapolitan rigatoni, their lasagne col pesto, and their mushroom ravioli. I don't think you can go wrong with any of their pasta dishes.

Based on what you've learned in Chapter 3, is this argument deductive or inductive? How can you tell?

Tess: Are there any good Italian restaurants in town?

Don: Yeah, Luigi's is pretty good. I've had their Neapolitan rigatoni, their lasagne col pesto, and their mushroom ravioli. I don't think you can go wrong with any of their pasta dishes.

Inductive.

The argument is an inductive generalization, which is a common pattern of inductive reasoning. Also, the conclusion does not follow with strict necessity from the premises.

I wonder if I have enough cash to buy my psychology textbook as well as my biology and history textbooks. Let's see, I have $200. My biology textbook costs $65 and my history textbook costs $52. My psychology textbook costs $60. With taxes, that should come to about $190. Yep, I have enough.

Is this argument deductive or inductive? How can you tell?

I wonder if I have enough cash to buy my psychology textbook as well as my biology and history textbooks. Let's see, I have $200. My biology textbook costs $65 and my history textbook costs $52. My psychology textbook costs $60. With taxes, that should come to about $190.

Yep, I have enough.

Deductive.

This argument is an argument based on mathematics,which is a common pattern of deductive reasoning. Plus,the conclusion follows necessarily from the premises.

Mother: Don't give Billy that brownie. It contains walnuts, and I think Billy is allergic to walnuts. Last week he ate some oatmeal cookies with walnuts, and he broke out in a severe rash.

Father: Billy isn't allergic to walnuts. Don't you remember he ate some walnut fudge ice cream at Melissa's birthday party last spring? He didn't have any allergic reaction then.

Is the father's argument deductive or inductive? How can

you tell?

Mother: Don't give Billy that brownie. It contains walnuts, and I think Billy is allergic to walnuts. Last week he ate some oatmeal cookies with walnuts, and he broke out in a severe rash.

Father: Billy isn't allergic to walnuts. Don't you remember he ate some walnut fudge ice cream at Melissa's birthday party last spring? He didn't have any allergic reaction then.

Inductive.

The father's argument is a causal argument, which is acommon pattern of inductive reasoning. Also, theconclusion does not follow necessarily from thepremises. (Billy might have developed an allergic reaction to walnuts since last spring.)

John is an agnostic. It necessarily follows that he doesn't believe in God.

Is this argument deductive or inductive? How can you tell?

John is an agnostic.

It necessarily follows that he doesn't believe in God.

Deductive.

This argument is an argument by definition, which is acommon pattern of deductive inference. Also, the phrase"it necessarily follows that" is a deduction indicatorphrase. Also, the conclusion follows from the premises.

Larry: Do you think Representative Porkmeister will be re-elected?

Norman: I doubt it. Porkmeister's district has become more conservative in recent years. Porkmeister is a liberal Democrat, and 63% of the registered voters in his district are now Republicans.

Is this argument deductive or inductive? How can you tell?

Larry: Do you think Representative Porkmeister will be re-elected?

Norman: I doubt it. Porkmeister's district has become more conservative in recent years. Porkmeister is a liberal Democrat, and 63% of the registered voters in his district are now Republicans.

Inductive.

This argument is both a statistical argument and a predictive argument, which are two common patterns ofinductive reasoning. Also, the conclusion does not follownecessarily from the premises.

If Buster walked to the game, then he didn't drive to the game. Buster didn't drive to the game.

Therefore, Buster walked to the game.

Is this argument deductive or inductive? How can you tell?

If Buster walked to the game, then he didn't drive to the game. Buster didn't drive to the game. Therefore, Buster walked to the game.

[This is the end of the tutorial] X

Deductive.

This argument is a hypothetical syllogism, which is acommon pattern of deductive reasoning.

Note, however, that the conclusion does not follow logically fromthe premises. (Maybe Buster rode his bike to the game, forexample.) The argument commits the fallacy of "affirming theconsequent."