To give an analogy is to claim that two distinct things are alike or similar in some respect. Here are two examples :

  • Capitalists are like vampires.
  • Like the Earth, Europa has an atmosphere containing oxygen.

The analogies above are not arguments. But analogies are often used in arguments. To argue by analogy is to argue that because two things are similar, what is true of one is also true of the other. Such arguments are called "analogical arguments" or "arguments by analogy". Here are some examples :

  • There might be life on Europa because it has an atmosphere that contains oxygen just like the Earth.
  • This novel is supposed to have a similar plot like the other one we have read, so probably it is also very boring.
  • The universe is a complex system like a watch. We wouldn't think that a watch can come about by accident. Something so complicated must have been created by someone. The universe is a lot more complicated, so it must have been created by a being who is a lot more intelligent.

Analogical arguments rely on analogies, and the first point to note about analogies is that any two objects are bound to be similar in some ways and not others. A sparrow is very different from a car, but they are still similar in that they can both move. A washing machine is very different from a society, but they both contain parts and produce waste. So in general, when we make use of analogical arguments, it is important to make clear in what ways are two things supposed to be similar. We can then proceed to determine whether the two things are indeed similar in the relevant respects, and whether those aspetcs of similarity supports the conclusion.

So if we present an analogical argument explicitly, it should take the following form :

(Premise 1) Object X and object Y are similar in having properties Q1 ... Qn.
(Premise 2) Object X has property P.
(Conclusion) Object Y also has property P.

Before continuing, see if you can rewrite the analogical arguments above in this explicit form.

§1. Analogical arguments and induction

It is sometimes suggested that all analogical arguments make use of inductive reasoning. This is not correct. Consider the explicit form of analogical arguments above. If having property P is a logical consequence of having properties Q1 ... Qn, then the analogical argument will be deductively valid. Here is an example :

(Premise 1) X and Y are similar in that they are both isosceles triangles (an isosceles triangle is a triangle with two equal sides).
(Premise 2) X has two equal internal angles.
(Conclusion) Y has two equal internal angles.

Of course, in such a situation we could have argued for the same conclusion more directly :

(Premise 1) Y is an isosceles triangles.
(Premise 2) Every isosceles triangle has two equal internal angles.
(Conclusion) Y has two equal internal angles.

What this shows is that :

  • Some good analogical arguments are deductively valid.
  • Sometimes we can argue for a conclusion more directly without making use of analogies. This might reveal more clearly the reasons that support the conclusion.

Of course, analogical arguments can also be employed in inductive reasoning. Consider this argument :

  • This novel is supposed to have a similar plot like the other one we have read, so probably it is also very boring.

This argument is of course not deductively valid. Just because the plot of novel X is similar to the plot of a boring novel Y, it does not follow logically that X is also boring. Perhaps novel X is a good read despite an unimpressive plot because its pace is a lot faster and the story telling is more gripping and graphic. But if no such information is available, and all we know about novel X is that its plot is like the plot of Y, which is not very interesting, then we would be justified in thinking that it is more likely for X to be boring than to be interesting.

§2. Evaluating analogical arguments

So how should we evaluate the strength of an analogical argument that is not deductively valid? Here are some relevant considerations :

  • Truth : First of all we need to check that the two objects being compared are indeed similar in the way assumed. For example, in the argument we just looked at, if the two novels actually have completely different plots, one being an office romance and the other is a horror story, then the argument is obviously unacceptable.
  • Relevance : Even if two objects are similar, we also need to make sure that those aspects in which they are similar are actually relevant to the conclusion. For example, suppose two books are alike in that their covers are both green. Just because one of them is boring does not mean that the other one is also boring, since the color of a book's cover is completely irelevant to its contents. In other words, in terms of the explicit form of an analogical argument presented above, we need to ensure that having properties Q1, ... Qn increases the probability of an object having property P.
  • Number : If we discover a lot of shared properties between two objects, and they are all relevant to the conclusion, then the analogical argument is stronger than when we can only identify one or a few shared properties. Suppose we find out that novel X is not just similar to another boring novel Y with a similar plot. We discover that the two novels are written by the same author, and that very few of both novels have been sold. Then we can justifiably be more confident in concluding that X is likely to be boring novel.
  • Diversity : Here the issue is whether the shared properties are of the same kind or of different types. Suppose we have two Italian restaurants A and B, and A is very good. We then find out that restaurant B uses the same olive oil in cooking as A, and buys meat and vegetables of the same quality from the same supplier. Such information of course increases the probability that B also serves good food. But the information we have so far are all of the same kind having to do with the quality of the raw cooking ingredients. If we are further told that A and B use the same brand of pasta, this will increase our confidence in B further still, but not by much. But if we are told that both restaurants have lots of customers, and that both restaurants have obtained Michelin star awards, then these different aspects of similarities are going to increase our confidence in the conclusion a lot more.
  • Disanalogy : Even if two objects X and Y are similar in lots of relevant respects, we should also consider whether there are dissimilarities between X and Y which might cast doubt on the conclusion. For example, returning to the restaurant example, if we find out that restaurant B now has a new owner who has just hired a team of very bad cooks, we would think that the food is probably not going to be good anymore despite being the same as A in many other ways.

§3. Analogical arguments in morality

Analogical arguments occur very frequently in discussions in law, ethics and politics. In a very famous article, "A Defense of Abortion", written in 1971, philosopher Judith Thomson argues for a woman's right to have an abortion in the case of unwanted pregnancy using an analogy where someone woke up one morning only to find that an unconscious violinist being attached to her body in order to keep the violinist alive. Thomson argues that the victim has the right to detach the violinist even if this would bring about the violinist's death, and this also means that a woman has the right to abort an unwanted baby in certain cases. For further discussion on the role of analogy in moral reasoning, see this article.

Evaluate these arguments from analogy. See if you can identify any aspects in which the two things being compared are not relevantly similar :

  1. We should not blame the media for deteriorating moral standards. Newspapers and TV are like weather reporters who report the facts. We do not blame weather reports for telling us that the weather is bad. answer
  2. Democracy does not work in a family. Parents should have the ultimate say because they are wiser and their children do not know what is best for themselves. Similarly the best form of government for a society is not a democractic one but one where the leaders are more like parents.

  3. "Wives, submit yourselves to your own husbands, as unto the Lord. For the husband is the head of the wife, even as Christ is the head of the church." - St. Paul, Ephesians 5:22. answer
  4. In the early 17th century, astronomer Francesco Sizi argued that there are only seven planets: "There are seven windows in the head, two nostrils, two ears, two eyes and a mouth; so in the heavens there are two favorable stars, two unpropitious, two luminaries, and Mercury alone undecided and indifferent. From which and many similar phenomena of nature such as the seven metals, etc., which it were tedious to enumerate, we gather that the number of planets is necessarily seven."