Tutorial

Logical Strength

Logical strength is the degree of support that the premises confer on a conclusion--the degree to which the premises, if true, make it likely that the conclusion is true as well. The stronger an argument is, the tighter the relationship between its premises and conclusion; the weaker it is, the looser the relationship.
Example:
Suppose that Lenny Lightfinger is accused of stealing Mary Mobil's car. Compare the following two bodies of evidence:

1. Lenny Lightfinger was once convicted of auto theft; therefore he stole Mary Mobil's car.

2. Lenny Lightfinger was seen by several witnesses entering Mary's car and driving it away, the car was later found one block from his home, and his fingerprints were on the steering wheel; therefore, he stole her car.

For both 1 and 2, let's assume the premises are all true. Nevertheless, it is clear that the premise in 1 provides very little evidence for the conclusion; it might justify a slight suspicion of Lenny, but nothing more; 1 is an extremely weak argument.
In 2, on the other hand, we have very strong evidence for Lenny's guilt. The conclusion that he was the thief is the only conclusion that seems consistent with the evidence.

Comprehension Questions

1. Each pair of arguments has the same conclusion. Determine which one has the greater logical strength.
(1) My husband, like all the members of his family, is a compulsive talker, because he drinks too much coffee and, thus, has too much caffeine in his system.
(2) My husband, like all the members of his family, is a compulsive talker, because of something in his genes.

a) (1) is stronger

b) (2) is stronger

2. Each pair of arguments has the same conclusion. Determine which one has the greater logical strength.
(1) Chronic Fatigue Syndrome is just the result of a yuppie’s overactive imagination.
(2) Chronic Fatigue Syndrome is the result of an overactive immune system that gets activated and then doesn't turn off.

a) (1) is stronger

b) (2) is stronger

3. Each pair of arguments has the same conclusion. Determine which one has the greater logical strength.
(1) The mass extinction of dinosaurs at the end of the Jurassic occurred not all at once, but in steps over millions of years. This was due to the fact that there was either a change in climate or epidemics made possible by the emergence of land bridges.
(2) The mass extinction of dinosaurs at the end of the Jurassic occurred not all at once, but in steps over millions of years. This was due to a sudden disaster that occurred at this time.

a) (1) is stronger

b) (2) is stronger

Assumed Premises

1.	Lenny Lightfinger was once convicted of auto theft; therefore he stole Mary Mobil's car.
2.	Lenny Lightfinger was seen by several witnesses entering Mary's car and driving it away, the car was later found one block from his home, and his fingerprints were on the steering wheel; therefore, he stole her car.

1.	Each pair of arguments has the same conclusion. Determine which one has the greater logical strength. (1) My husband, like all the members of his family, is a compulsive talker, because he drinks too much coffee and, thus, has too much caffeine in his system. (2) My husband, like all the members of his family, is a compulsive talker, because of something in his genes.

	a)	(1) is stronger
	b)	(2) is stronger


2.	Each pair of arguments has the same conclusion. Determine which one has the greater logical strength. (1) Chronic Fatigue Syndrome is just the result of a yuppie’s overactive imagination. (2) Chronic Fatigue Syndrome is the result of an overactive immune system that gets activated and then doesn't turn off.

	a)	(1) is stronger
	b)	(2) is stronger


3.	Each pair of arguments has the same conclusion. Determine which one has the greater logical strength. (1) The mass extinction of dinosaurs at the end of the Jurassic occurred not all at once, but in steps over millions of years. This was due to the fact that there was either a change in climate or epidemics made possible by the emergence of land bridges. (2) The mass extinction of dinosaurs at the end of the Jurassic occurred not all at once, but in steps over millions of years. This was due to a sudden disaster that occurred at this time.

	a)	(1) is stronger
	b)	(2) is stronger