In which of the following does the conclusion necessarily follow from the premises?

PHILOSOPHICAL ARGUMENTS

A Crash Course

The Dialectic = the attempt to develop a sustained pattern of argument in which the implications of different positions are drawn out and interact with each other.

Argument = a series of statements (premises) intended to establish the truth/plausibility of another statement (conclusion).

The following words/phrases usually indicate that a premise is about to appear: "because," "since," "for," "for the reason that," etc.

The following words/phrases usually indicate that a conclusion is about to appear: "therefore," "thus," "and so," "consequently," "necessarily," "hence," "it follows that," "for that reason," etc.

Generally, there are two types of argument: the inductive argument and the deductive argument.

Inductive Argument: This type of argument argues from repeated instances to a general conclusion. This is the form of most scientific arguments.

Example: This swan is white, that swan is white, . . . all ten thousand of the swans I have seen are white. Therefore, all swans are white.

Example: Shoemaker has failed all the students he has had who missed more than six class periods in a semester. Therefore, he will fail me if I miss more than six class periods this semester.

Deductive Argument: This type of argument involves a proof by logical inference, whereby a conclusion is arrived at by means of applying rules of logic to the premises. If a deductive argument is done correctly, then if the premises are true, the conclusion must be true as well. This is the type of argument most often used in philosophy, and it is the domain of formal logic.

The standard form of deductive arguments is the syllogism = a deductive argument in which a conclusion is inferred from two premises.

1. All human beings are mortal.
2. Britney Spears is a human being.

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3. Thus, Britney Spears is mortal.

How to Evaluate Deductive Arguments:

1. Do the premises in fact support the conclusion? This is a question of logical validity, i.e., is the argument valid? To say that a deductive argument is valid is to say that the rules of inference have been applied correctly, so that if the premises are true, then the conclusion must also be true. Thus, a valid argument is an argument that has followed the rules of logic correctly. A valid argument may have a true or false conclusion, depending upon the truth of its premises. An invalid argument may also have a true or false conclusion. All that it means to say that an argument is invalid is to say that that argument provides no logical support for its purported conclusion, as the conclusion does not follow from the premises.

   Example of a valid argument:

   1. All CSUN students are geniuses.
   2. President Bush is a CSUN student.
   
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   3. Thus, President Bush is a genius.

   In this example, both premises 1 and 2 are false. But the argument is still valid, because if the premises were true, the conclusion (#3) would necessarily be true. It remains doubtful whether #3 is true anyway.

   Another kind of valid argument involves "if . . . then" statements, as follows:

   Example #2 of a valid argument:

   1. If a movie lasts longer than 2-1/2 hours, then it is a bad movie.
   2. The Godfather lasts longer than 2-1/2 hours.
   
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   3. Thus, The Godfather is a bad movie.

   However, if the second premise is an instance of the "then" clause, instead of the "if" clause, the argument becomes invalid.

   Example of an invalid argument (the "Gilligan's Island" argument):

   1. If my charm necklace is lucky, then it won't snow on the island.
   2. It does not snow on the island.
   
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   3. Therefore, my charm necklace is lucky.


2. Are the premises clear? This is a question about the meanings of the terms used. Sometimes the same word can have several different senses. Or sometimes it may be unclear just what is meant at all by a particular word. If we are unsure of what a certain word means within an argument, we must suspend judgment on its overall effectiveness.

   For example, suppose someone put as a premise the following claim: "All persons have souls." We first have to find out precisely what is meant both by the word "persons" and by the word "soul" before we can go on to evaluate the truth of the premise. Similarly for other controversial terms, like "pornography," "God," "good/bad," etc.

3. Are the premises (fairly obviously) true? This is a question of the soundness of an argument. A sound argument is one that is valid and has all true premises. The conclusion of a sound argument, therefore, must necessarily be true as well.

   Example of a sound argument:

   1. All the casinos in Las Vegas serve beer.
   2. The Excalibur is a casino in Las Vegas.
   
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   3. Thus, the Excalibur serves beer. (Woo hoo!)
   
   

The anti-abortion argument:

1. ____________________________________


2. ____________________________________


3. Therefore, ____________________________

next:  Logic Examples

When a conclusion follows from the premises?

What kind of argument is where the conclusion necessarily follows from the premises?

How do you show a conclusion in a premises?

Is the conclusion guaranteed by the premises?