A logical fallacy or a spurious argument in which it is wrongly assumed that two things, facts, or phenomena that are similar or the same in certain respects are also the same in others.
An argument by analogy is a logical inference based on the following pattern:
A is similar to B
B has property C
Therefore, A [probably] also has property C.
For example:
An apple is similar to a pear.
Pears are tasty fruits.
Therefore, apples are [probably] also tasty fruits.
This example also highlights the limitations of this method: whilst apples and pears do share some characteristics, they also differ in many others. Similarly, one could compare them to quinces, which, although they resemble both apples and pears in appearance, cannot really be described as “tasty” – at least not when eaten raw.
The inserted word “probably” is therefore important, even though it is usually omitted in everyday speech: analogical reasoning yields, at best, probable conclusions, not certain ones (Abduction).
In essence, an analogy is therefore more akin to a heuristic, in that it often provides a “good enough” explanation, which may not, however, be valid in all circumstances.
There are no “guaranteed” rules governing the circumstances under which such an inference leads to a correct result. However, it helps if the common traits are relevant to the inference; in other words, if the trait being inferred follows causally from the known common traits, or at least could do so. This is referred to as a “strong” analogy.
This type of reasoning becomes fallacious precisely when this constraint is disregarded; that is, when a similarity in one aspect leads to the assumption of similarities in others, even though this is not appropriate.
For example:
The orbits of the planets in the solar system are similar to those of electrons in an atom.
Electrons can “jump” from one orbit to another.
Therefore, the planets can also jump from one orbit to another.
However, one should not assume that all false analogies are as obviously nonsensical as in such examples. Here is another example showing how anthropomorphism based on a false analogy can lead to a false conclusion:
You can chat with this AI just as you would with a real person.
People have emotions and can act intentionally.
Therefore, this AI also has emotions and can act intentionally.
For more information on this topic, see: Anthropomorphisation.
The term “false equivalence” refers to a specific type of false analogy in which two things, ideas, or circumstances are presented as morally, logically or factually equivalent, even though there are significant differences between them. In this context, it is primarily used as a form of rhetorical fallacy.
Treacherously, such analogies often seem quite plausible and reasonable at first glance, as in the following example:
A national budget is to a state what a household budget is to a private household.
A private household cannot run up new debts indefinitely, but must at some point start saving and reducing its debts.
Therefore, the national budget must also make savings in order to reduce debt.
Especially when it comes to debt, however, there are important differences between private and public finances. The latter, for example, have a virtually ‘infinite’ time horizon, whereas private households should aim to be debt-free by the time they reach retirement age and, ideally, should even have built up some savings.
In general, it is of course perfectly reasonable to manage public finances prudently and economically. In this case, however, the simplistic equation of household finances with those of the state has led to far-reaching political decisions (such as the so-called debt brake, or Schuldenbremse in German), for which a more fact-based discussion grounded in economic principles – and a little fewer simplistic slogans – would certainly have been more appropriate.