Lecture Notes by Christopher Lay

Los Angeles Pierce College

Department of History, Philosophy, and Sociology

Mathew Van Cleave's 2016 Introduction to Logic and Critical Thinking

https://open.umn.edu/opentextbooks/BookDetail.aspx?bookId=457

Chapter 3 "Evaluating Inductive Arguments and Probabilistic and Statistical Fallacies," § 3 "Analogical Arguments" 

"In an argument from analogy, we note that since some thing x shares similar properties to some thing y, then since y has characteristic A, x probably has characteristic A as well." 

Analogical arguments often begin by showing how two things share some qualities.  The next step is to show other qualities are also shared. 

1) Establish shared, non-controversial similarities between X and Y. 

2) Show how controversial target feature is found in X. 

3) Assert that controversial target feature must also be found in Y. 

Conditions of Strength

"[A]n argument from analogy is [inductively] strong only if the following two conditions are met:" 

A) There must be enough relevant similarities

B) There should not be any significant, relevant dissimilarities 

Relevant Similarities

"The characteristics of the two things being compared must be similar in relevant respects to the characteristic cited in the conclusion."

"This is a key condition for any good argument from analogy: the similar characteristics between the two things cited in the premises must be relevant to the characteristic cited in the conclusion." 

The more the better:  "better arguments from analogy will invoke more relevant similarities between the things being compared in the analogy." 

No Significant, Relevant, Dissimilarities 

"There must not be any relevant disanalogies between the two things being compared." 

Handling Analogical Arguments

Differences: object to an analogical argument by showing "that there are relevant differences between the two things being compared in the analogy." 

Similarities: object to an analogical argument by showing that there are not enough relevant similarities shared by the two things being compared in the analogy.

Example Analogical Argument

1. "Bob chose to have a luxury item for himself rather than to save the life of a child." 

2. "'We' regularly choose having luxury items rather than saving the life of a child."

3. "What Bob did was morally wrong."

4. "Therefore, what we are doing is morally wrong as well."

If "'we'" are relevantly similar to Bob, then this argument seems rather convincing. 

Exercise 24

"Evaluate the following arguments from analogy as either strong or weak. If the argument is weak, cite what you think would be a relevant disanalogy." 

1. "Every painting by Rembrandt contains dark colors and illuminated faces, therefore the original painting that hangs in my high school is probably by Rembrandt, since it contains dark colors and illuminated faces."

"Evaluate the following arguments from analogy as either strong or weak. If the argument is weak, cite what you think would be a relevant disanalogy." 

2. "I was once bitten by a poodle. Therefore, this poodle will probably bite me too."

"Evaluate the following arguments from analogy as either strong or weak. If the argument is weak, cite what you think would be a relevant disanalogy." 

3. "Every poodle I’ve ever met has bitten me (and I’ve met over 300 poodles). Therefore this poodle will probably bite me too."

"Evaluate the following arguments from analogy as either strong or weak. If the argument is weak, cite what you think would be a relevant disanalogy." 

4. "My friend took Dr. VanCleave’s logic class last semester and got an A. Since Dr. Van Cleave’s class is essentially the same this semester and since my friend is no better a student than I am, I will probably get an A as well."