Evaluating Arguments

EVALUATING ARGUMENT: VALIDITY AND SOUNDNESS

An argument is a combination of statements. Some of these statements are premises or assumptions and some are conclusions. Premises of the argument state reasons for believing that the conclusion(s) of the argument is true. That is, the premises support  the conclusion(s) of the argument.

 

SOME EXAMPLES OF ETHICAL ARGUMENTS   

The Benefits Argument (a form of consequentialism):

1. (MP) If we can benefit someone, without harming anyone else, we ought to do so.
2. Transplanting the organs would benefit the other children without harming Baby Theresa
3. Therefore, we ought to transplant the organs. [from 1 & 2]

The Argument  That We Should Not Use People (Merely) As Means (a form of deontology):

1. (MP) It is wrong to use people (merely) as means to other people’s ends.
2. Taking Theresa’s organs would be using her (merely) as a means to benefit other children.
3. Therefore, it should not be done. [from 1 & 2]

The Argument From The Wrongfulness of Killing

1. (MP) It is wrong to kill one person to save another.
2. Taking Theresa’s organs would be killing her to save another.
3. Therefore, it would be wrong to kill Theresa in order to harvest her organs. [from 1 & 2]

The Slippery Slope Argument

1. If we accept any sort of mercy killing, we will have stepped onto a “slippery slope” down which we will inevitably slide, and in the end all life will be held cheap.
2. It is wrong to allow for this kind of “cheapening” of human life.
3. Hence, Tracy should not have been killed. [from 1 & 2]

 

FEW OTHER EXAMPLES OF PHILOSOPHICAL ARGUMENTS

The Argument from Evil

1. If God exists, then this world was created by an omnipotent, omniscient, and omnibenevolent (OO&O) being.
2. If this world was created by an OO&O being, then this world contains no evil. (there is nothing bad in this world)
3. This world contains some evil.
4. Therefore, God does not exist. [from (1)‑(3)]

A Skeptical Argument

1. If someone knows something, then she has certainty about it.
2. If someone is certain about something, then she cannot be mistaken about it.
3. We can be mistaken about anything.
4. So, no one is ever certain of anything. [from (2) and (3)]
5. herefore, no one knows anything. [from (1) and (4)]

Argument from Choice

1. Sometimes we do what we choose to do.
2. If sometimes we do what we choose to do, then sometimes we act freely.
3. Therefore, sometimes we act freely. [from (1) and (2)]  

 

EVALUATING ARGUMENTS

There are two questions to ask:

1. Does the conclusion follow from the premises? That is, is the argument valid?

2. Are the premises true (or at least justified)? Or, is the argument sound (or, at least, strong)?

 

VALIDITY

• In a valid argument, the conclusion follows from the premises.

• In other words, if the premises are (or were) true, then the conclusion must also be true.

• That is, it is impossible for the premises of the valid argument all to be true and its conclusion to be false.

In order to determine whether an argument is valid or not, ask yourself: Supposing that the premises are or were true (whether they really are or not), must the conclusion be true? If the answer is yes, then the argument is valid. If the answer is no, then the argument is invalid.

Notice: valid arguments may have false premises and false conclusions.

Some examples of valid arguments: All examples above are valid arguments. Here are a few more:

1. Mahatma Gandhi is a Texan. (F)
2. All Texans wear sombreros. (F)
3. Thus, Gandhi wears a sombrero. (from 1, 2) (F)

 

1. All hens quack. (F)
2. My friend's pet, Lulu, is a hen. (T)
3. Hence, Lulu quacks. (from 1, 2) (F)

 

1. All native Texans are at least one inch tall. (T)
2. Stefan is a native Texan. (F)
3. Stefan is at least one inch tall. (from 1, 2) (T)

 

1. All women are Romans. (F)
2. Caesar was a woman. (F)
3. Caesar was a Roman. (from 1, 2) (T)

 

Notice, these examples illustrate the fact that a valid argument may have all combinations of truth a falsity of premises and conclusion with one exception: if the premises of a valid argument are true, then so is its conclusion. It is never th case that an argument is valid and has all true premises but its conclusion is false.

 

SOUNDNESS

• A sound argument is both  valid; and  all of its premises are true.

• Sound arguments prove that their conclusions are true. They are proofs.

 
Some examples of sound arguments

1. All men are mortal. (T)
2. Socrates is a man. (T)
3. Hence, Socrates is mortal. (from 1,2) (T)

 

1. No vegetarians eat met. (T)
2. Gandhi was a vegetarian. (T)
3. Gandhi did not eat meat. (from 1,2) (T)

 

SOME QUESTIONS FOR REVIEW: 

1) All of the premises of any valid argument must be true. (T/F)

 

2) All of the premises of any sound argument must be true. (T/F)

 

3) The conclusion of a sound argument is true. (T/F) 

 

4) An invalid argument may have all true premises and a true conclusion. (T/F)

HINT: Notice that to say that an argument is valid means only that the conclusion follows from the premises (and not that the premises and conclusion are true). Conversely, to say that all the premises and the conclusion of some argument are true does not mean that the conclusion follows from the premises. Also, consider this argument:

1) All men are mortal.

2) Stef is a man.

So, 3) Stef teaches philosophy.

Both premises are true and the conclusion is also true. But does this conclusion follow from the premises? So, is this argument valid?

 

5) An invalid argument may have all true premises and a false conclusion. (T/F)

 

6) Some valid arguments are not sound. (T/F)

 

7) Some sound arguments are not valid. (T/F)

 

8) The conclusion of a valid argument must be true. (T/F)