Weak Analogy


Taxonomy: Logical Fallacy > Informal Fallacy > Weak Analogy3



A is like B.
B has property P.
Therefore, A has property P.


Efforts to ban chlordane assailed

WASHINGTON (AP)--The only exterminator in Congress told his colleagues Wednesday that it would be a short-sighted move to ban use of chlordane and related termiticides that cause cancer in laboratory animals.

Supporters of the bill, however, claimed that the Environmental Protection Agency was "dragging its feet" on a chemical that could cause 300,000 cancers in the American population in 70 years.

"This bill reminds me of legislation that ought to be introduced to outlaw automobiles" on the grounds that cars kill people, said Rep. Tom DeLay, R-Texas, who owns an exterminating business.

EPA banned use of the chemicals on crops in 1974, but permitted use against termites because the agency did not believe humans were exposed. Chlordane does not kill termites but rather drives them away.6



An argument by analogy is one with an analogy among its premisses―see the Form, above. Since Weak Analogy is not a formal fallacy, but an informal one, there is nothing logically wrong with the form. Instead, what makes an analogical argument strong or weak is the strength or weakness, the relevance or irrelevance, of the analogy in its premisses. If the analogy is too weak or irrelevant to the conclusion of the argument, then it will commit the fallacy of Weak Analogy.


Some arguments from analogy are based on analogies that are so weak that the argument is too weak for the purpose to which it is put. Such arguments are those that commit the fallacy of weak analogy. How strong an analogy needs to be depends upon the context in which it occurs, and the use that it is intended to serve. Thus, in the absence of other evidence, and as a guide to further research, even a very weak analogy may be strong enough. So, while the strength of an argument from analogy depends upon the strength of the analogy in its premisses, it is not solely determined by that strength. An analogical argument commits the fallacy of weak analogy when, and only when, the analogy in its premisses is too weak to serve its purpose.


Here are two important points about analogy:

  1. No analogy is perfect, that is, there is always some difference between analogs. Otherwise, they would not be two analogous objects, but only one, and the relation would be one of identity, not analogy. For this reason, no analogy is true.
  2. There is always some similarity between any two objects, no matter how different. For example, Lewis Carroll once posed the nonsense riddle: "How is a raven like a writing desk?"7 The point of the riddle was that they're not; alike, that is. However, to Carroll's surprise, some of his readers came up with clever solutions to the supposedly unsolvable riddle, for instance: "Because Poe wrote on both."8 For this reason, no analogy is false.

Analysis of the Example:

Representative DeLay attempts to argue against a bill banning chlordane by comparing it to a bill banning automobiles, but this analogy is very weak. Here are some of the relevant differences:


  1. Robert J. Gula, Nonsense: A Handbook of Logical Fallacies (2002), pp. 108-10, 163.
  2. Madsen Pirie, How to Win Every Argument: The Use and Abuse of Logic (2006).
  3. This is the common name for this fallacy; see, for instance: David Hackett Fischer, Historians' Fallacies: Toward a Logic of Historical Thought (1970), Chapter 9. However, this name is misleading, since analogies are neither true nor false. Instead, they come in degrees from near identity to extreme dissimilarity. It's more accurate to say that a close similarity between two things is a "strong" analogy, and a distant similarity is a "weak" one. For this reason, I call it the fallacy of weak analogy.
  4. T. Edward Damer, Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments (3rd edition, 1994), pp. 101-3.
  5. Howard Kahane, Logic and Contemporary Rhetoric: The Use Reason in Everyday Life (5th edition, 1988), pp. 82-5.
  6. Associated Press, June 25th, 1987.
  7. Alice in Wonderland, Chapter 7.
  8. Martin Gardner, The Annotated Alice (1960), p. 95.