Inductive logic begins with specfic examples and reaches a general conclusion.Deductive logic takes a general conclusion and applies it to a specific situtation.
For instance, in the days after 9/11, analysts said that terrorist attacks have never been successful in achieving their aims. For this reason, America would overcome the effects of the terrorists' act.
In maintaining this, the analysts were using deductive logic. They were taking a general historic fact and applying it to a specific situtation. If we were to diagram the deductive argument, it would look like this:
Major Premise: Historically, no terrorist attack has achieved its goal.
Minor Premise: The September 11th attack was a terrorist attack.
Conclusion: The September 11th attack will fail to achieve its goal.
The truth of such an argument depends on the truth of the major premise. If someone can provide a historical instance where terrorism did achieve its aims, then we'd have to revise our position. Similarly, we know that just because a terrorist act has not achieved its aim yet does not mean that no terrorist act ever will. Finally, if someone argues that the specific incident was not a terrorist action, we'd have to revise our position.
The quality of a deductive argument depends on the reader's willingness to accept our major premise. Additionally, our essay will have to prove that the minor premise is a subset of the major premise--in this case that the 9/11 attack was, in fact, a terrorist act. If the major premise is accepted as being true and the minor premise is accepted as being true, the conclusion must be true.
And that's the problem with deductive reasoning. There are not many statements that everyone will regard as true. Certainly, most of us will accept scientific statements as true, but the truth of science changes all the time. Consider this instance of deductive reasoning:
Major Premise: Ships that sail far out to sea fall off the edge of the world
Minor Premise: Columbus says he'll sail far out to sea.
Conclusion: Columbus will fall off the edge of the world!
We can pretty much agree that the major premise is false, so we can also agree that the conclusion is false. Certainly, the minor premise is true. But the minor premise is not a subset of the major premise because the major premise is not true.
This example shows the weakness of deductive reasoning. We have trouble coming up with a major premise that is true at all times. Conditions change, knowledge changes, beliefs change. Thus, while we can still use deductive strategies to structure our arguments, we have to provide the evidence, the supports, to convince our readers that the major premise is true, that the minor premise is a subset of the major premise, and that the conclusion, therefore, is true.
If we employ a deductive strategy to argue for a tax increase for public education, we might arrange our argument in this way:
Major Premise: Cities that increase taxes to support public education benefit in a number of ways.
To simplify the argument, writers seldom discuss the major premise. Instead, because they feel that it will be recognized as the truth by their readers, they use a form of the kind of argument we've been discussing. This form is called an enthymeme, and it assumes that the audience will supply the major premise without our having to: because, our argument maintains, readers will assume that cities investing in public education benefit, we can assume that it's a given, rather than going to extremes to prove it before we even get to our argument.Minor Premise: Our city should increase taxes to support public education.
Conclusion: Our city will benefit in a number of ways.
Using a deductive strategy, the essay would probably be structured something like this:
Introduction: The minor premise would be outlined as a proposition to be discussed: "Our city should raise taxes to support public education." The major premise would not be developed here, but the writer would allude to the generally accepted truth that increased funding of public schools benefits the cities in other ways.
Body: The body of the essay would try to articulate those ways, and here, the writer would provide evidence that other cities that had increased educational funding had enjoyed the very effects being considered. Cities that increased funding experienced increased test scores among their children, population growth as businesses moved in, and an increased tax base that made tax reductions possible. The body also might have a pathetic appeal, an appeal to the taxpayers that would be designed to add emotion to an essay that might otherwise read like the minutes of a business meeting.
Conclusion: Much as the inductive argument, the conclusion would restate the premise, summarize its support, and call for the voters to act.
While we can see the similarities as well as the differences in the deductive and inductive strategies, both employ both general and specific information to support their central theses. Further, both ways of organizing information reflect the ways that we experience and learn.
However, both systems have limitations. The conclusion in inductive logic depends on an inductive leap, a summing up of what the specific incidents mean. Deductive logic depends on finding a major premise that will be accepted as true by the average reader. Given the political, cultural, ethnic, and economic diversity of our culture, finding such a major premise can be difficult.