PHI103 Mastery

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What is a sound argument?

*A valid argument with all true premises. A sound argument is a valid argument in which all the premises are true. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. The answer "a valid argument in which it is impossible to have true premises and a false conclusion" is redundant, like saying that the argument is valid and valid. A valid argument, by definition, cannot have true premises and a false conclusion. An argument in which it is possible to have true premises and a true conclusion does not ensure that it is valid (which would require that it is not possible for it to have true premises and a false conclusion). For a more in-depth discussion of this topic see section 3.1.

What is the relationship between a valid argument form and its instances?

*All instances of a valid argument form are valid arguments. A valid argument is a substitution instance of a valid argument form. A valid argument form is one with no possible substitution instance having true premises and a false conclusion. An invalid argument form is one that does have substitution instances with true premises and a false conclusion.

Which of these nonsense arguments have a valid form?

*All quigs are bleebs. All bleebs are twogs. Therefore, all quigs are twogs. *All quigs are bleebs. No bleebs are twogs. Therefore, no quigs are twogs. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. Therefore, in a valid argument, if the premises are true, then the conclusion must be true as well. Since validity is a matter of form, even non-sense arguments can be valid. If all quigs are bleebs and no bleebs are twogs, it follows that a quig (being a bleeb) cannot be a twog. Similarly, if all quigs are bleebs and all bleebs are twogs, then it follows that all quigs are twogs. However, there is nothing in "all quigs are bleebs" and "all quigs are twogs" that guarantees that no gleebs will be twogs. For a more in-depth discussion of this topic see section 3.2.

Which of the following could be true of a valid argument?

*All the premises are false, and the conclusion is false. *All the premises are true, and the conclusion is true. *All the premises are false, but the conclusion is true. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. Therefore, in a valid argument, if the premises are true, then the conclusion must be true as well. Validity has to do with the relationship between the premises and conclusion, not about whether the premises or conclusion are actually true. For an argument to be valid means that it is not possible for the premises to all be true and the conclusion false, so the only thing that a valid form guarantees is that the premises are not all true and the conclusion false. Here is an example of a valid argument with false premises and a true conclusion: All dogs are reptiles. All reptiles are mammals. Therefore, all dogs are mammals. For a more in-depth discussion of this topic see section 3.1.

What is a valid argument?

*An argument in which the truth of the premises guarantees the truth of the conclusion *An argument in which it is impossible to have true premises and a false conclusion. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. In other words, it is an argument in which it is impossible for the premises to be true while the conclusion is false. Notice that the definition of valid does not say anything about whether the premises are actually true, just whether the conclusion could be false if the premises were true. For a more in-depth discussion of this topic see section 3.1.

Consider the argument: "The paranormal has been thoroughly debunked, so ghosts are not real." What additional premises would be needed to make this argument valid?

*Ghosts are paranormal. *Anything debunked is not real. In order to make the argument valid, we need to link the information in the premise to the conclusion. In this case, we would need the information that ghosts are paranormal and that anything thoroughly debunked is not real. That argument becomes: The paranormal has been thoroughly debunked.Anything thoroughly debunked is not real.Ghosts are paranormal.So, ghosts are not real. The other choices do not link the original premise to the conclusion. For a more in-depth discussion of this topic see the following video: deLaplante, K. (2013, January 31). What is a good argument?: The logic condition [video file]. Retrieved from https://youtu.be/bA8GuloqV60

Consider the argument:"Driving drunk is wrong because it puts people's lives at risk." What additional premise would be needed to make this argument valid?

*It is wrong to put people's lives at risk. The premise that makes this argument valid is the one that links the premise to the conclusion. To get from the premise that drunk driving puts people's lives at risk to the conclusion that it is wrong, we would need to claim that putting people's lives at risk is wrong. The full argument, in standard form, goes: Drunk driving puts people's lives at risk.It is wrong to put people's lives at risk.Therefore, drunk driving is wrong. The other choices do not link the original premise to the conclusion. For a more in-depth discussion of this topic see section 5.3.

Which of the following is always true of a sound argument?

*Since it is valid and its premises are true, its conclusion will be true as well. A sound argument must have a true conclusion. TRUE: If an argument is sound, then it is valid and has all true premises. Since it is valid, the argument is such that if all the premises are true, then the conclusion must be true.

Consider the argument: "There is no way the Spurs will win the championship. The Rockets are better." What additional premise is assumed?

*The Spurs would have to be better than the Rockets in order to win the championship. This argument is trying to prove that the Spurs will not win the championship. The fact that the Rockets are better is only relevant if the Spurs would have to beat them in order to win the championship. Otherwise the premises would not support the conclusion. So, the complete argument, in standard form (premises each on a separate line, above the conclusion, which is the claim the premises are trying to support) goes: The Rockets are better than the Spurs.The Spurs would have to be better than the Rockets to wih the championship.Therefore, the Spurs will not win the championship. "The Rockets are the superior team" is just paraphrasing the first premise above. It is not necessary that the Spurs have already lost to the Rockets. Nor is it implied that the Rockets will win the championship, just that the Spurs will not. For a more in-depth discussion of this topic see What is a Good Argument?: The logic condition [Video file]. How to Make a Sound Deductive Argument [Video file]

Regarding valid arguments, which of these are true?

*The conclusion is true if the premises are. *The truth of the premises would guarantee the truth of the conclusion. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. Therefore, in a valid argument, if the premises are true, then the conclusion must be true as well. However, this does not mean that the premises are actually true or that the conclusion is, only that if the premises were to be true, then the conclusion would be true as well. Here is an example of a valid argument with false premises and a false conclusion: All dogs are reptiles. All reptiles are green. Therefore, all dogs are green. For a more in-depth discussion of this topic see section 3.1.

Consider the argument: "You should not buy that stock because it is overpriced, and it is too risky." What additional premise would be needed to make this argument valid?

*You should not buy a stock that is overpriced and too risky. In order to make the argument valid, we need to link the information in the premises to the conclusion. To get the conclusion that you should not buy the stock, you would need to link the premise that it is overpriced and too risky to the conclusion that you should not buy it. The argument becomes: That stock is overpriced.That stock is too risky.You should not buy a stock that is overpriced and too risky.Therefore, you should not buy that stock. The other choices do not link the original premise to the conclusion. They may be relevant, but the specific link that we need is that you should not buy any stock that has the characteristics listed in the premises. For a more in-depth discussion of this topic see the following video: deLaplante, K. (2013, January 31). What is a good argument?: The logic condition [video file]. Retrieved from https://youtu.be/bA8GuloqV60

Take the argument form:All As are Bs.No Bs are Cs. Therefore, no As are Cs. Which of these is an instance of the given argument form?

All dogs are mammals. No mammals are reptiles. Therefore, no dogs are reptiles. An instance of the argument form is found by replacing the letters A, B, and C with specific examples. Replacing A with dogs, B with mammals, and C with reptiles yields the argument "All dogs (A) are mammals (B). No mammals (B) are reptiles (C). Therefore, no dogs (A) are reptiles (C)." For a more in-depth discussion of this topic see section 3.2.

Take the argument form:All As are Bs.Some Bs are Cs. Which of these is a counterexample to the validity of the argument form?

All dogs are mammals. Some mammals are cats. Therefore, some dogs are cats. A counterexample to the validity of an argument form is an instance of that form that leads from true premises to a false conclusion. This shows that the argument form is invalid. An counterexample to the validity of the given argument form is found in the argument: "All dogs (A) are mammals (B). Some mammals (B) are cats (C). Therefore, some dogs (A) are cats (C)." It is an instance of the given form, yet it has true premises and a false conclusion. For a more in-depth discussion of this topic see section 3.2.

Consider the argument: "All snakes are reptiles, so all snakes are mammals." What additional premise would be needed to make this argument valid?

All reptiles are mammals.

Which of the following can be called valid or sound?

Arguments In logic, the words valid and sound refer only to arguments. A sound argument is a valid argument in which all the premises are true. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. In informal logic, statements, declarative sentences, and claims are treated as synonymous. They make up the premises and conclusions of arguments, but only the full argument can be called valid or sound. Validity has to do with the quality of the reasoning used by the argument, not whether the premises or conclusion are true or false. For a more in-depth discussion of this topic see section 3.1.

What would it mean for an argument to be unsound?

Either it is invalid or at least one premise is false. An unsound argument is either an invalid argument or a valid argument with at least one false premise.

Consider the argument: "Capital punishment is unjust because it sometimes executes innocent people." What additional premise would best support the reasoning used?

Executing innocent people is unjust.

What can we say for sure about an argument with all true premises and a false conclusion?

It is invalid. A valid argument is one in which the truth of the premises absolutely guarantees the truth of the conclusion. In other words, it is an argument in which it is impossible for the premises to be true while the conclusion is false. So if the premises are, in fact, true while the conclusion is false, then we know that this is not a valid argument. For a more in-depth discussion of this topic see section 3.1.

What makes an argument deductive?

It presents itself as being valid.

Consider the argument: "He is a Republican. So he will not support this bill." What additional premise would be needed to make this argument valid?

No Republican will support this bill.

Consider the argument: "Rover is a dog. So he is not as fast as a horse." What additional premise would be needed to make this argument valid?

No dog is as fast as a horse.

Regarding valid arguments, which of these are true?

The argument can be valid even if the premises are not actually true. A valid argument is one in which the truth of the premises, if true, would guarantee the truth of the conclusion. This does not require that the premises are actually true, only that if they were all true, then the conclusion would have to be true as well.


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