I



Cogent Reasoning

a. The point of figuring out the structure of an argument is to determine whether it is cogent (good) or fallacious (bad). If it is cogent, we should be willing to accept its conclusion; if it is bad, we should not.

i. To be cogent, an argument has to satisfy three conditions. It must…

1. start with justified (warranted, believable) premises

a. This first criterion requires that we bring to bear whatever we already know or are justified in believing—our relevant background beliefs—to determine whether we should or should not accept the premises of an argument being evaluated.

b. Background beliefs can be categorized in many ways—for instance, into beliefs about facts and beliefs about values, including moral values.

c. In dealing with most social or political issues, it is important to separate claims that are factual from those that have to do with values, because these two different kinds of claims are defended, or justified, in very different ways.

i. The claim that a given state has a death penalty is verified by examining the relevant government records.

ii. The judgment that the death penalty is or isn’t justified for the commission of heinous crimes is verified by referring to a moral code one accepts or to subjective intuitions of the matter.

2. include all likely relevant information (Satisfying this extremely stringent requirement is usually beyond the ability of most of us most of the time, so the point is to come as close as possible to satisfying it, bearing in mind the seriousness of the problem to be solved and the cost—in time and money—of obtaining or recalling relevant information.)

a. This second criterion of cogent reasoning requires that we not pass over relevant information. In particular, it tells us to resist the temptation to neglect evidence contrary to what we want to believe.

b. Consider this argument: “We absolutely must start cutting down on the use of oil as an energy source. The World Resources Institute estimates that at the present rate of consumption, known reserves will be used up in just a bit more than 30 years.”

i. Even supposing the Institute’s estimate were on target, this argument still would not be cogent because it suppresses relevant information that most knowledgeable people have had all along and which the World Resources Institute experts also ought to have known about.

1. Known reserves continually increase anyway, due to the discovery of new oil fields, in spite of ever-rising consumption.

2. Prospecting for oil is expensive and chancy, so that when oil is a glut on the market, exploration slows down.

3. When reserves become lower relative to demand, exploration increases.

c. Arguments that neglect relevant evidence are not cogent.

d. Those who do not know about the overlooked information, or have simply neglected to bring it to bear, or do not have reason to suspect that relevant information is being suppressed are fair game for more knowledgeable and less principled operators.

3. be valid, or correct, which means that being justified in accepting its premises justifies acceptance of its conclusion—provided we know nothing else relevant to the conclusion. (Note that an argument containing one or more unjustified premises may still be cogent if it also has justified premises that support its conclusion.

a. The third criterion of cogent reasoning requires that the premises of an argument genuinely support its conclusion; or, as logicians would say, it requires that the argument be valid, or correct.

b. There are two fundamentally different ways in which premises may support conclusions.

i. The first way yields deductively valid arguments.

1. The fundamental property of a deductively valid argument is this: If all of its premises are true, then its conclusion must also be true because the claim asserted by its conclusion already has been stated in its premises, although usually only implicitly. Here is an example:

a. Everything made of copper conducts electricity. (Premise)

b. This wire is made of copper. (Premise)

c. This wire will conduct electricity. (Conclusion)

i. It would be contradictory to assert both of its premises and then deny its conclusion.

2. It is important to see that the deductive validity of an argument has nothing to do with whether its premises are true. Validity has to do with the nature of the connection between premises and conclusion, not the truth or believability of premises.

3. Determining that an argument is deductively valid tells us that if its premises happen to be true, then its conclusion must be true also—it does not tell us whether its premises are true.

ii. The second way yields inductively valid arguments.

1. These arguments, unlike deductive one, have conclusions that go beyond what is contained in their premises.

2. The idea behind a valid induction is that of learning from experience.

3. Valid inductions simply project regularities (patterns or resemblances).

4. We use inductive reasoning so frequently in everyday life that its nature generally goes unnoticed.

5. Induction increases possibility of error because the premises of a perfectly good induction may all be true and yet its conclusion may be false. Here’s an example:

a. A restaurant that has served excellent food many times in the past fails us on a special occasion.

6. Even the best “inductive leap” can lead us astray because the pattern noticed in our experience up to a given point may not turn out to be the exact pattern of the whole universe.

Deductively Valid Arguments and Its Forms (Structures)

Different arguments may have the same form, or structure, as do these two deductively valid arguments:

Argument 1

1. If it’s spring, then the birds are chirping.

2. It is spring.

3. The birds are chirping. (conclusion)

Argument 2

1. If a world government does not evolve soon, then wars will continue to occur.

2. A world government isn’t going to evolve soon.

3. Wars will continue to occur. (conclusion)

Deductively Valid Forms and Examples

Modus Ponens Form:

1. If A then B. (Premise) If it’s spring, then the birds are chirping.

2. A. (Premise) It is spring.

3. B. (Conclusion) The birds are chirping.

Modus Tollens Form:

1. If A then B. (Premise) If it is spring, then the birds are chirping.

2. Not B. (Premise) The birds aren’t chirping.

3. Not A. (Conclusion) It isn’t spring.

Hypothetical Syllogism Form:

1. If A then B. (Premise) If we successfully develop nuclear fusion power, then power will become cheap and plentiful.

2. If B then C. (Premise) If power becomes cheap and plentiful, then the economy will flourish.

3. If A then C. (Conclusion) If we successfully develop nuclear fusion power, then the economy will flourish.

Disjunctive Syllogism Form:

1. A or B. (Premise) Either Bush won the 1992 election or Clinton did.

2. Not A. (Premise) Bush didn’t win.

3. B. (Conclusion) Clinton did.

Other Forms:

1. No F’s are G’s. (Premise) No police officers accept bribes.

2. It’s false that some F’s are G’s. (Conclusion) It’s false that some police officers accept bribes.

1. All F’s are G’s. (Premise) All salamis are tasty.

2. If this is an F, then this is a G. (Conclusion) If this is a salami, then it is tasty.

1. All F’s are G’s. (Premise) All TV evangelists have high moral standards.

2. All G’s are H’s. (Premise) All who have high moral standards live up to those standards.

3. All F’s are H’s. (Conclusion) All TV evangelists live up to high moral standards.

1. All F’s are G’s.(Premise) All elective offices always tell the truth.

2. This is an F. (Premise) Bill Clinton is an elected official.

3. This is a G. (Conclusion) Bill Clinton always tells the truth.

1. All F’s are G’s. (Premise) All males are chauvinist pigs.

2. No G’s are H’s. (Premise) No chauvinist pigs are likeable.

3. No F’s are H’s. (Conclusion) No males are likeable.

1. No F’s are G’s. (Premise) No foreigners can be trusted.

2. Some H’s are F’s. (Premise) Some newborn babies are foreigners.

3. Some H’s are not G’s. (Conclusion) Some newborn babies cannot be trusted.

Deductive Invalidity

Any argument that does not have a deductively valid form is said to be deductively invalid.

Assignment: Invent deductively valid arguments for each of the forms.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download