Two Kinds of Reasoning
Notes 37-45
Philosophy 60
Dr. Wells

GENERAL FEATURES OF AN ARGUMENT:

Argument must have two parts, the premise that is intended to give a reason for the conclusion, and the conclusion, which should follow from the premise.

Statements or claims made without are not arguments.

The following are not arguments:

“God exists”

“God exists. That is as plain as the nose on your face.

“God exists and if you don’t believe it, you will go to Hell.” (Scare tactics)

“I think God exists because I was raised a Baptist.” This explains why the speaker believes in God but does not give a reason for “God exists.”

The following is an argument:

“God exists because something must have caused the universe.”

This gives a reason or premise for the conclusion that God exists.

Conclusions of One Argument Can Be Used as the Premise for Another.

Example:

Premise—Jose did not study for Critical Thinking
Conclusion: Jose got an ‘F.’

Jose got an ‘F.’
Jose will not be able to transfer Critical Thinking to San Jose St.

Jose will not be able to transfer Critical Thinking to San Jose St.
Jose will not be admitted to San Jose St.

This is an example of a chain of reasoning that is made up of three separate arguments.

The above can be compressed into an argument with an unstated premise:

Jose got an ‘F’ in Critical Thinking
Jose will not be admitted to San Jose State.

The unstated premise is that Jose must be able to transfer Critical Thinking to San Jose State in order to be admitted to San Jose State.

Conclusion Indicators

The following words usually suggest that a conclusion will follow:

Thus
Therefore
Hence
This shows that
This suggests that
Consequently
So
Accordingly
This implies that
This proves that

Jose got an ‘A’ in Critical Thinking.
Consequently, Jose will meet a requirement for admittance to San Jose State.

Premise Indicators

The following words usually suggest that a premise is to follow:

Since
For
In view of
This is implied by

Unstated premises are used more frequently than unstated conclusions. A premise can be unstated when it is obvious and does not require that it be stated.
As was mentioned above, the example given in the “compressed” above has an unstated premise.

TWO KINDS OF ARGUMENTS

Deductive

A successful deductive argument ‘proves’ or ‘demonstrates’ its conclusion.

Perhaps the most famous deductive argument is:

All Greeks are mortal.
Aristotle is a Greek
Aristotle is mortal.

If the premises are true the conclusion absolutely must follow.

A deductive argument either is absolutely true or it is a failed argument.

In other words, the conclusion of a successful deductive argument cannot be false--or another term meaning the same thing--is certain.

However, there are two kinds of successful deductive arguments:
1) Valid
2) Sound

A valid deduction argument is one where the conclusion follows from the premises but one or more of the premises may be false.

This may appear to be a strange sort of argument. How can an argument be false and yet be a good argument. An argument with a false argument would have to be false, right?

But there is a distinction that can be made between whether an argument is true and whether an argument is logical.

Consider the following argument:

All Presidents of the U.S. have three arms.
Obama is the President of the U.S.
President Obama has three arms.

Obviously the conclusion of this argument is false.
But the argument is logical.
IF the premises were true the conclusion would follow. In other words internally the argument is successful.

An argument that is logical but may be false is called valid.

Let’s return to the above argument
There is a false premise—that all Presidents of the U.S. have three arms. If this premise is corrected—‘all U.S. Presidents have two arms’ the conclusion would become ‘Obama