Some Notes on Section 1.1

Arguments, Premises, and Conclusions

Logic is about ARGUMENTS. People use arguments in reasoning. Informally, arguments can be seen as offering reasons for believing things. Some reasons are good reasons for believing things, and some are not so good.

Sentences and Statements

The things we give reasons for, and the reasons we give for them, can be expressed in SENTENCES. More precisely, they can be expressed in sentences of a certain type: sentences that are either true or false.

Some sentences in English (or any other natural language) have the property that it makes sense to call them true or false. Consider these examples:

Today is Sunday.
There is a cockroach in my raspberry frappé
Two plus two is four.
Two plus two is five.
2 + 2 = 5.
It will rain tomorrow.
It will rain at 2:00 PM in College Station, Texas, on March 13, 2050.
It rained in College Station, Texas, on March 13, 1950.
The number of people alive at this moment is an even number.
The set of all subsets of an infinite set is of higher cardinality than the set itself.

These are all sentences that it at least makes sense to call true or false in a way that it does not make sense to call any of these sentences true or false:

What time is it?
I'd like dry white toast and two fried chickens, please.
Who was the seventh President of the United States?
I wish this class were over!
Get out of my room and leave me alone.
I hope you die a prolonged and miserable death.

Try saying "That's true" or "That's not so" about each of these to see the difference.

How do you tell whether a sentence is a statement?

How do you recognize a sentence? That's part of what it is to understand a language. It's very complex, but children can do a pretty good job of this in their native languages by the age of five or so. Here are some sentences:

Vita brevis, ars longa.
Minä puhun Englantia, mutta en puhu paljon Suomea.
J'aime beaucoup la logique, mais je déteste les logiciens.

Even in your own language, it's sometimes not obvious whether something is a sentence. Try these examples.

The dog died.
The dog the cat bit died.
The dog the cat the possum ate bit died.
The dog the cat the possum the baby befriended ate bit died.
The dog the cat the possum the baby the armadillo corrupted befriended ate bit died.

Or even:

The dog the dog the dog the dog the dog bit bit bit bit died.
Buffalo buffalo buffalo buffalo buffalo.

Premises and Conclusions

Here are some arguments:

Hieronymus is a Republican.
Therefore, Hieronymus is a conservative.

Farquhar has either a cat or a dog.
Yesterday, I saw him with a dog.
Consequently, he doesn't have a cat.

My kitchen is ten feet long.
In addition, the width of my kitchen is thirteen feet.
As a result, the area of my kitchen is 150 square feet.

Tabby's a cat and cats are mammals, so Tabby doesn't have feathers.

If a year is divisible by four, then it is a leap year unless it is divisible by 100 and not divisible by 400.
2000 is divisible by four.
2000 is also divisible by 100.
However, 2000 is divisible by 400.
Therefore, 2000 had 366 days.

Smith is a philosophy professor.
Smith is therefore an imbecile.
After all, only an imbecile would be a philosophy professor.

Each of these arguments consists of sentences, and in fact of sentences of the kind that must be true or false. In addition, one sentence in each of them is distinguished in a certain way. One way to describe the distinction is to say that that sentence is what the argument is trying to prove, or the point of the argument, while all the other sentences are offered as support for that sentence, or reasons for accepting it. We will use the term CONCLUSION for the sentence that's distinguished in this way, and we will call each of the other sentences a PREMISE.

So, how do we tell when an argument is going on, and how do we tell which sentence is the conclusion? Though we'll have a little more to say about that later, we're going to define ARGUMENT in an extremely broad way: an argument is just some sentences (the PREMISES) and another sentence (the CONCLUSION). Formally:

An ARGUMENT is a pair of things:

a set of sentences, the PREMISES

a sentence, the CONCLUSION

On this definition, all of the following are arguments:

Today is Thursday.
If today is Thursday, then tomorrow is Friday.
Therefore, tomorrow is Friday.

Today is Thursday.
Therefore, tomorrow is Friday.

(You don't have to have more than one premise)

Today is Thursday.
Today is Wednesday.
Therefore, tomorrow is Friday.

(It doesn't have to be a good argument)

Today is Thursday.
Today is the sixth of the month.
Six is a perfect number.
The population of the United States is approximately 300,000,000.
Therefore, there is no life on Mars.

(The premises don't have to have anything to do with the conclusion)

Therefore, two plus two equals four.

(Strictly speaking, you can have an empty set of premises)

Valid and Invalid Arguments

Here is the single most important definition in this course:

An argument is VALID if and only if it is necessary that if all its premises are true, then its conclusion is true.

A valid argument is an argument in which there is a certain relationship between its premises and its conclusion. That relationship concerns the truth values of the premises and conclusion

"Truth value" is a convenient way of saying "truth or falsehood". Arguments are composed of sentences that are either true or false, so every such sentence has a truth value. Its truth value is "true" if the sentence is true and "false" if the sentence is false (you're not surprised?).

Returning to validity, to say that an argument is valid is to say that the truth values of its premises and its conclusion are related in a certain way: IF the premises are ALL true, THEN the conclusion MUST be true.

Since this is easy to misunderstand, let's spend some time on it. First, it does not say that in an argument, if the premises are true then the conclusion must be true. Instead, it gives the criterion for a valid argument.

How can we tell whether the conclusion must be true if all the premises are true? Well, what's necessary is what can't possibly be otherwise, so if something can possibly be otherwise, then it's not necessary. (That, by the way, was an argument). So, to tell whether an argument is valid, we can:

First, suppose or imagine that all the premises are true (regardless of whether or not they actually are true).
Next, see if we can imagine the conclusion being false under those circumstances.
If we can think of a way for the premises all to be true and the conclusion to be false at the same time, then we know that the argument is INVALID.
If we are certain that there is no way for the premises all to be true at the same time that the conclusion is false, then we know that it is VALID.

This sounds like it depends rather a lot on how good we are at thinking up ways that things might be. In fact, we're going to develop some precise ways of doing that for certain arguments as this course proceeds. Let's take a quick look now at how you might proceed, however. Here's an argument:

In order to pass Professor Abelard's course, Porphyry must either (1) have an average of at least C on each of the four exams and the final or (2) have a passing grade on all the exams and the final and submit an acceptable term paper.
Porphyry has received grades of D on the first three exams.
Each exam, including the final, is weighted equally.
Although Porphyry wrote a term paper that would have been acceptable, a group of brigands has stolen it and will not return it to him unless he pays a ransom of $5000.
Porphyry does not have $5000, and he does not know anyone who does.
Being an honorable person, Porphyry does not steal.
The term paper must be turned in by tomorrow to be acceptable.
It is impossible for Porphyry to write another term paper by tomorrow.
Therefore, Porphyry cannot pass Professor Abelard's course.

Sound Arguments

A SOUND argument is just a valid argument with true premises, that is:

An argument is SOUND if and only if it is valid and all its premises are true.

What else can you conclude about an argument on the basis of that definition?

Exercise 1.1

All of these can be answered on the basis of the definitions already given.

i*. Every premise of a valid argument is true: NO: Whether an argument is valid depends on what would happen if the premises WERE all true, non on whether they actually are all true.
ii*. Every invalid argument has a false conclusion: NO: If the premises of a valid argument are not all true, then nothing follows about whether the conclusion is true or not.
iii*. Every valid argument has exactly two premises: NO: An argument (valid or otherwise) may have any number of premises, including only one (or even including zero)
iv*. Some valid arguments have false conclusions: YES: The only thing that can't happen with a valid argument is having the conclusion false when the premises are all true.
v*. Some valid arguments have false conclusions despite having premises that are all true: NO: This almost exactly contradicts the definition of 'valid'.
vi*. A sound argument cannot have a false conclusion: YES: If a valid argument can't have a false conclusion when its premises are all true, and if a sound argument is a valid argument with true premises, then this follows right away.
vii*. Some sound arguments are invalid: NO: Part of the definition of SOUND is VALID ARGUMENT.
viii*. Some unsound arguments have true premises: YES: can you say which ones?
ix*. Premises of sound arguments entail their conclusions: YES: See the definition of ENTAILS.
x*. If an argument has true premises and a true conclusion, then it is sound.: We can talk about this in class

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