Supporting evidence (induction and deduction)

Induction

Induction is the type of reasoning that moves from specific facts to a general conclusion. When you use induction in your paper, you will state your thesis (which is actually the conclusion you have come to after looking at all the facts) and then support your thesis with the facts. The following is an example of induction taken from Dorothy U. Seyler’s Understanding Argument:

Facts:

There is the dead body of Smith. Smith was shot in his bedroom between the hours of 11:00 p.m. and 2:00 a.m., according to the coroner. Smith was shot with a .32 caliber pistol. The pistol left in the bedroom contains Jones’s fingerprints. Jones was seen, by a neighbor, entering the Smith home at around 11:00 p.m. the night of Smith’s death. A coworker heard Smith and Jones arguing in Smith’s office the morning of the day Smith died.

Conclusion: Jones killed Smith.

Here, then, is the example in bullet form:

• Conclusion: Jones killed Smith

• Support: Smith was shot by Jones’ gun, Jones was seen entering the scene of the crime, Jones and Smith argued earlier in the day Smith died.

• Assumption: The facts are representative, not isolated incidents, and thus reveal a trend, justifying the conclusion drawn.

Deduction

When you use deduction in an argument, you begin with general premises and move to a specific conclusion. There is a precise pattern you must use when you reason deductively. This pattern is called syllogistic reasoning (the syllogism). Syllogistic reasoning (deduction) is organized in three steps:

1. Major premise

2. Minor premise

3. Conclusion

In order for the syllogism (deduction) to work, you must accept that the relationship of the two premises lead, logically, to the conclusion. Here are two examples of deduction or syllogistic reasoning:

Socrates

1. Major premise: All men are mortal.

2. Minor premise: Socrates is a man.

3. Conclusion: Socrates is mortal.

Lincoln

1. Major premise: People who perform with courage and clear purpose in a crisis are great leaders.

2. Minor premise: Lincoln was a person who performed with courage and a clear purpose in a crisis.

3. Conclusion: Lincoln was a great leader.

So in order for deduction to work in the example involving Socrates, you must agree that 1) all men are mortal (they all die); and 2) Socrates is a man. If you disagree with either of these premises, the conclusion is invalid. The example using Socrates isn’t so difficult to validate. But when you move into more murky water (when you use terms such as courage, clear purpose, and great), the connections get tenuous.

For example, some historians might argue that Lincoln didn’t really shine until a few years into the Civil War, after many Union losses to Southern leaders such as Robert E. Lee.

The following is a more clear example of deduction gone awry:

1. Major premise: All dogs make good pets.

2. Minor premise: Doogle is a dog.

3. Conclusion: Doogle will make a good pet.

If you don’t agree that all dogs make good pets, then the conclusion that Doogle will make a good pet is invalid.

The following is a more clear example of deduction gone awry:

1. Major premise: All dogs make good pets.

2. Minor premise: Doogle is a dog.

3. Conclusion: Doogle will make a good pet.

If you don’t agree that all dogs make good pets, then the conclusion that Doogle will make a good pet is invalid.