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