INTRODUCTION TO FORMAL REASONING

Traditionally, arguments or reasoning are divided into two types: deductive and inductive.

Deductive reasoning tries to prove the truth of its conclusions beyond any doubt. Inductive reasoning tries to show that their conclusions are plausible or likely or probable to be true given the premises.

Often, the two are distinguished from each other as follows: deductive reasoning is that which flows from general to specific, and inductive reasoning is that which flows from specific to general. However, this is not always true as shown in the examples below:

Example 1:

Three is a prime number.Five is a prime number.Seven is a prime number.Therefore, all odd number between two and eight are prime numbers.

(This is an example of a deductive argument/reasoning but it flows from specific premises to a general conclusion.)

Example 2:

All of Stephen King’s previous books have been best-sellers.His next book will probably be a best seller.

(This is an example of an inductive argument/reasoning but it flows from a general premise to a specific conclusion.)

Therefore, it is not the pattern of particularity or generality that makes an argument either inductive or deductive. It is the type of support the premises are claimed to provide for the conclusion that determines whether an argument is deductive or inductive.

The following table shows the key differences between deductive and inductive reasoning:

Deductive reasoning claims that - Inductive reasoning claims that -If the premises are true, the conclusion is certainly true

If the premises are true, the conclusion is probably true

The conclusion follows necessarily from the premises

The conclusion follows probably from the premises

The premises provide conclusive evidence for the truth of the conclusion

The premises provide good (but not conclusive) evidence for the truth of the conclusion

The truth of the premises guarantees the truth of the conclusion

The truth of the premises makes the truth of the conclusion likely

So, the question to ask is this: Does the conclusion follow with strict necessity from the premises or does the conclusion simply follow the premises with a degree of probability?

Study of law employs both inductive and deductive reasoning, such as in the following instances:

1. Legal principles to be used in a particular case are determined by inductive (inductive analogy); and

2. Relevant legal principles/laws are applied to the facts of a particular case by deduction.

Fundamental in the study and application of formal logic are the following parts of an argument:

1. Premisesa. Major – statement of lawb. Minor – statement of fact

2. Conclusion – application of the law to the facts