Deductive Reasoning
Newton's Law says that everything that goes up must come down. Therefore, if I kick the ball up, it will come down (to see how one might arrive at this same conclusion through inductive reasoning, click here).
Short explanation: Deduction is the logical process of drawing specific conclusions from general premises. Whereas induction uses what is known about some particulars to draw general conclusions, deduction uses what is known to be true in general to draw specific conclusions.
Longer Explanation
Consider the following mathematical expression:
A = B.
C = A.
Therefore, C = B.
Simple enough, yes? If it is true that A = B and C = A, then it also has to be true that C = B. It has to be. There's simply no room for argument. Look at it this way:
1/2 is .5
one-half is 1/2
Therefore, one-half is .5
Now consider the same "equation" but now with concepts instead of variables or numbers:
Premise: All men are mortal.
Premise: Mike is a man.
Conclusion: Therefore, Mike is mortal.
This is the same as the above mathematical expression:
Premise: All men(A) are mortal(B). [A is B]
Premise: Mike(C) is a man(A). [C is A]
Conclusion: Therefore, Mike(C) is mortal(B). [C is B]
This form of reasoning is called a deductive syllogism, and it works by applying a general principle (the first premise) to a specific case (the second premise) to generate a conclusion. Here are some more deductive forms:
Premise (general rule or principle): All parents should prepare breakfast for their children.
Premise (specific case): Jane is a parent.
Conclusion: Therefore, Jane should prepare breakfast for her children.
Premise: Either we are going to Hawaii or Utah for vacation.
Premise: We didn't go to Hawaii.
Conclusion: Therefore, we went to Utah.
Truth In; Truth Out--Untruth In; Untruth Out
Here's the thing about deductive reasoning: unlike the conclusions arrived at via induction which are only probable, conclusions arrived at via deduction are guaranteed 100% true---IF THE PREMISES ARE TRUE AND INTERPRETED LOGICALLY.
And there's the rub. It's a big "if." The rock solid reliability of a deductive conclusion depends, in large part, on the accuracy and truth of the premises. Take our earlier example:
A = B.
C = A.
Therefore, C = B.
The conclusion is absolutely guaranteed true as long as A = B and C = A. If one or both of those is false (if, for example, A did not equal B but equaled X instead), then the conclusion is likewise false. Look at it this way:
1/2 is .55
one-half is 1/2
Therefore, one-half is .55
The conclusion is false because the first premise is false (i.e. 1/2 is NOT .55).
Or think about it this way:
Premise: Gentlemen prefer blondes.
Premise: Scott is a gentleman.
Conclusion: Scott prefers blondes.
The conclusion is shaky because the first (and possibly the second) premise is unreliable.
Because a deductive argument is vulnerable in its premises, that is usually where it is attacked. Opponents to an argument will work hard to cast doubt the reliability of one or both of the premises because they know that doing so will cast doubt on the conclusion. Imagine that your opponent in a debate offered this argument:
Premise: Medical doctors should be legally permitted to provide any treatment proven to relieve patients' suffering.
Premise: Marijuana has been proven to relieve the suffering of glaucoma patients.
Conclusion: Medical doctors should be permitted to provide marijuana to glaucoma patients.
We could respond to such an argument by presenting our own reasons to show why medical doctors should not be permitted to prescribe marijuana to glaucoma patients (and that may be the right thing to do). However, because two opposing statements cannot both be equally true, we should also look for flaws in our opponent's argument. We do that by looking for flaws in one of the premises. Is it really true that medical doctors should be legally permitted to prescribe ANY treatment? Have the criteria for "proof" been clearly established and, if so, has marijuana been definitively proven to relieve suffering? Who does our opponent mean by "medical doctors?" Does the prescribing doctor have to be board certified? Does he/she have to be a patient's primary care provider? Does the vagueness of the argument in these areas create weakness? Can we cast doubt on the reliability of the premises by questioning these things?
How about this one:
Premise: Parents who subject their children to harm are guilty of child abuse.
Premise: Smoking in confined spaces in the presence of children is harmful.
Conclusion: Parents who smoke in confined spaces in the presence of their children are guilty of child abuse.
At first glance, both premises look pretty solid. If that's the case, then the conclusion is also pretty solid. But let's look very closely and test where that first premise might take us:
Premise: Parents who subject their children to harm are guilty of child abuse.
Premise: Breathing in the air of Los Angeles (or eating too much McDonald's or going to beach without sunscreen, etc.) is harmful.
Conclusion: Parents who allow their children to breathe LA air (or feed their kids Happy Meals everyday) are guilty of child abuse.
Even though both premises in both syllogisms are "true," there's a problem. The second premise in each syllogism is okay but not great. Astute arguers are going to rigorously debate the definition of "confined spaces" with us, and they are going to insist that we specify what we mean by "smoking"--does it mean one puff or a pack--and what we mean by children--is a 17 year-old a "child"?. But an even bigger problem exists in the first premise. It is so vague that the argument squirrels a bit out of control. If we stick with the first premise as it is, we may find ourselves having to accept a number of conclusions we find objectionable (as exemplified in the second syllogism involving sunscreen and McDonald's). This is a common problem in deductive arguments, and the problem can be eliminated (or more often simply reduced) by refining the premise with specifics. Maybe we could do something to define the degree of harm we're concerned with. And maybe we could add a word like "routinely" or "habitually" in front of the word subject or a word like "preventable" in front of harm (and we'll have to define what we mean by these words). The more specific we can make the premise, the more control we'll have over the argument and the fewer "holes" our opponents can poke in it. Perhaps a better argument is the following:
Premise: Parents who knowingly and routinely subject their young children (ages 0 to 13 years old) to significant but preventable physical harm are guilty of child abuse.
Premise: Smoking for extended periods of time in confined spaces (i.e. spaces with limited ventilation or no immediate means of escape, such as an automobile) in the presence of children is preventable and significantly harmful.
Conclusion: Parents who smoke in confined spaces in the presence of their children are guilty of child abuse.
The specifics narrow the argument and make it more manageable. As you can see, it's still not an air-tight, obvious argument like the ones at the top of the page, and it will take a lot of work to establish the truth of the premises. But if we can do so, we will convince readers of our conclusion. Similarly, in a value-based argument like the following one, we may never be able to definitively prove the truth of the premises, but with clear, detailed thinking we can convince readers to accept the premises:
Premise: Reducing awareness of social differences is a desirable goal for public grade schools.
Premise: A uniform dress code would help achieve this goal.
Conclusion: Therefore, students should be required to dress uniformly.
If the form of the argument is broken, the argument won't work...
For a deductive argument to work, the form must be "valid"--that is, the ideas must come together logically. Usually, this means that the subject of the first premise appears in the second premise, and the subject of the second premise appears in the conclusion. Look at an earlier syllogism again:
Premise: All men are mortal. ("men" is the subject)
Premise: Mike is a man. ("Mike" is the subject)
Conclusion: Therefore, Mike is mortal.
Often, a deductive argument falls apart because the arguer does something like this:
Premise: All men are mortal.
Premise: Yogi Bear is mortal.
Conclusion: Yogi Bear is a man.
The premises are true, but the form is invalid because the subject of premise #1 doesn't appear in premise #2. If it did (i.e. if we had "Yogi Bear is a man"), we'd quickly see that premise#2 is faulty. Look at the following:
Premise: All murderers have ears.
Premise: All Methodists have ears.
Conclusion: All Methodists are murderers.
Again, the subject of the first premise doesn't appear in the second and the logic falls apart. This is a common problem when one is beginning to learn deduction. It is also common for beginning deducers to create a second premise that doesn't act as a bridge between the first premise and the conclusion. For example,
Premise: Parents who knowingly and routinely subject their young children (ages 0 to 13 years old) to significant but preventable physical harm are guilty of child abuse.
Premise: Parents can cause harm to their children in a number of different ways.
Conclusion: Conclusion: Parents who smoke in confined spaces in the presence of their children are guilty of child abuse.
The second premise is related to first premise thematically, but it doesn't allow for a logical progression to the conclusion.
Examples of Deductive Arguments
Take Home Quiz: Type your responses to the following
1. What conclusion follows from these two premises:
Premise: Activities that divert terrorists away from US soil make Americans safer.
Premise: The war against Iraq has attracted terrorists away from the US to Iraq.
Conclusion: ?
2. What conclusion follows from these two premises:
Premise: Activities that build resentment against the US make Americans less safe.
Premise: The war against Iraq has intensified anti-US sentiment around the globe.
Conclusion: ?
3. What major premise provides the basis for this minor premise and conclusion:
Premise: ?
Premise: The Olympic games possess the characteristics named above more so than any other sporting event.
Conclusion: The Olympic games are the most authentic sporting events.
4. What minor premise provides a bridge from the major premise to the conclusion in the following:
Premise: The 2nd amendment guarantees US citizens the right to own arms.
Premise: ?
Conclusion: Therefore, US citizens have the legal right to own military assault weapons.
5. After reading the essay "Warrior Culture," provide the deductive syllogism:
Premise: ?
Premise: ?
Conclusion: ?