As described by Govier "An argument based on analogy begins by using one case (usually agreed on and relatively easy to understand) to illuminate or clarify another (usually less clear). It then seeks to justify a conclusion about the second case on the basis of considerations about the first. The grounds for drawing the conclusion are the relevant similarities between the cases, which show a commonality of structure."
The general form of an analogical argument is:
P1. A has properties p, q, r
P2. B has properties p, q, r
P3. A has property s
C. Therefore B has property s
P1. John's brother and parents smoked two packs of cigarettes a day and ate fatty foods.
P2. John smoked two packs of cigarettes a day and ate fatty foods.
P3. John's brother and parents all died prematurely of heart attacks.
C. Therefore, John will probably die prematurely of a heart attack.
The strength or weakness of an analogical argument depends upon a number of considerations:
Similarity - Verify that the properties proposed as being shared among the comparison group do indeed exist. As analogical arguments are rarely actually presented in the form above, it may even be necessary to first list just how it is assumed the comparison groups are similar. Here is a simple example. "John is like Mike. Mike is smart. Therefore John must be smart". In this example none of the assumed similarities between John and Mike have been presented. Before the argument can carry any weight these similarities must be listed and verified.
Relevance - The more relevant the comparison properties are to the target properties, the stronger the argument. Here is an example of an analogical argument which lacks relevance. "Book A and Book B both have a hardbound cover, pages, words on the pages and numbers at the bottom of the pages. Book A is a boring story. Therefore we can assume that Book B has a boring story." Though I have given a number of similar properties between Book A and Book B, none of these properties are relevant and thus do nothing to increase the probability that Book B is boring.
Number - The more properties two things share in common with each other, the stronger the argument. This is based on the notion that the more two things are alike, the more likely they also share the property stated in the conclusion. As stated above, relevance plays a key role in determining how much weight these similarities are given.
Disanalogy - Relevant disanalogies or dissimilarities must also be considered when determining the strength or weakness of an analogy. For example if I say, "I have known three people who have had surgery at this hospital with the same surgeon and they have all turned out successfully. Therefore Jane's surgery will also be a success." But what if the three success stories all had minor surgery and Jane is scheduled for a high risk procedure? This of course would be a very relevant disanalogy.
Critical Thinking Web: Analogical Arguments
PHIL 1500: Dr. Donald Nute: Unit 3: Analogical Arguments