An analogical argument is an argument in which one concludes that two things are alike in a certain respect because they are alike in other respects.
Non-argument analogies are often used in cases in which one thing (X) is understood and another (Y) is not, to conclude something about Y. These are typically called explanatory comparisons/analogies. For example, a person might attempt to explain email by saying that it is like mail sent to a post office box. Just as mail is delivered to the PO box and you go to pick it up, email is delivered to your email in box and your software “goes” and picks it up.
Analogical arguments are often used in cases in which one thing (X) is accepted/seen as plausible and another (Y) is not, to get the audience to accept Y or see it as plausible. For example, a person might start with something everyone accepts, such as the fact that if a person has the blood cut off to her brain for too long, she’ll suffer brain damage. The person could then make an analogy: the education system is like the “brain” of society and money is the blood of this brain. Finally, the person might conclude by claiming that cutting off money to the education system will damage society.
Analogies can range from the very literal, such as drawing an analogy between humans and the rats used to test a new medicine, to the metaphorical, such as the blood and money example given above.
Analogical arguments are extremely common. In addition to being used in everyday life, they are commonly used in law and medicine. For example, when a lawyer argues from precedent, she is most likely using an analogical argument: In case Y the judged made ruling X, my case is like Y, so the judge should make ruling X. Doctors also make extensive use of analogical arguments. For example, they draw analogies between what they observed in medical school and what they are observing in a specific patient: this patient’s condition closely resembles the case of poison ivy I saw in medical school, so she has been exposed to poison ivy.
Strictly presented, an analogy will have three premises and a conclusion. The first two premises establish the analogy by showing that the things in question are similar in certain respects. The third premise establishes the additional fact known about one thing and the conclusion asserts that because the two things are alike in other respects, they are alike in this additional respect as well.
Although people generally present analogical arguments in a fairly informal manner, they have the following logical form:
Premise 1: X has properties P,Q, and R.
Premise 2: Y has properties P,Q, and R.
Premise 3: X has property Z as well.
Conclusion: Y has property Z.
X and Y are variables that stand for whatever is being compared, such as chimpanzees and humans or blood and money. P, Q, R, and Z are also variables, but they stand for properties or qualities, such as having a heart or being essential for survival. The use of P, Q, and R is just for the sake of the illustration-the things being compared might have many more properties in common.
An example of an analogy presented in strict form is as follows:
Premise 1: Rats are mammals and possess a nervous system that includes a developed brain.
Premise 2: Humans are mammals possess a nervous system that includes a developed brain.
Premise 3: When exposed to Nerve Agent 274, 90% of the rats died.
Conclusion: If exposed to Nerve Agent 274, 90% of all humans will die.
Standards of Assessment
The strength of an analogical argument depends on three factors. To the degree that an analogical argument meets these standards it is a strong argument.
First, the more properties X and Y have in common, the better the argument. For example, in the example given above rats and humans have many properties in common. This standard is based on the common sense notion that the more two things are alike in other ways, the more likely it is that they will be alike in some other way. It should be noted that even if the two things are very much alike in many respects, there is still the possibility that they are not alike in regards to Z. This is why analogical arguments are inductive.
Second, the more relevant the shared properties are to property Z, the stronger the argument. A specific property, for example P, is relevant to property Z if the presence or absence of P affects the likelihood that Z will be present. Using the example, above, the shared properties are relevant. After all, since nerve agents work on the nervous system, the presence of a nervous system makes it more likely that something will be killed by such agents. It should be kept in mind that it is possible for X and Y to share relevant properties while Y does not actually have property Z. Again, this is part of the reason why analogical arguments are inductive.
Third, it must be determined whether X and Y have relevant dissimilarities as well as similarities. The more dissimilarities and the more relevant they are, the weaker the argument. In the example above, humans and rats do have dissimilarities, but most of them are probably not particularly relevant to the effects of nerve agents. However, it would be worth considering that the size difference might be relevant-at the dosage the rats received, humans might be less likely to die. Thus, size would be a difference worth considering.
Premise 1: Attacking your next-door neighbors, killing them and taking their property is immoral.
Premise 2: War involves going into a neighboring country, killing people and taking their property.
Conclusion: So, war is immoral.
War and violent theft share many properties: intrusion, violence, killing, and taking the property of others. War and violent theft also share relevant properties: violence, killing, and taking of property are relevant to moral assessment.
However, there are relevant dissimilarities. For example, war often takes place between mutual antagonists. This relevant difference can be developed, perhaps ironically, in another analogical argument: it could be argued that while it would be immoral for a person to just randomly attack neighbors, just as a boxing match between two opponents is morally acceptable, a war between two willing opponents would be morally acceptable as well.