• What is an analogy?
• Types of analogy
• Examples of analogies
• Argument by analogy
• Analogy in law
• Analogy in Biology

We explain what analogy is, its use in argumentation, classification and examples. Also, analogy in law and biology.

The metaphor of the black sheep proposes an analogy between a human group and a herd.

What is an analogy?

The analogy is a kind of reasoning or expressive mechanism of the language. It consists of comparing or relating various references: objects, reasons or ideas, to point out general and particular characteristics in common, in order to justify the existence of a property in one of them.

This is, more easily said, that an analogy is a form of comparison of a referent with another or others that are similar, but not identical, in order to be able to attribute some characteristics determined by the set. The term comes from ancient Greek Ana-, "Reiteration" and logos, "Word" or "thought".

The reasoning of the analogy can be represented by a general formula that would be "A is to B as C is to D". The metaphor, simile, homology and comparison are rhetorical figures which can be considered as analogies.

The analogy as a procedure is commonly used in different areas of the thought, from language and logic formal, until right, the philosophy and even more specific areas, such as biology, where specific events are designated with the same term.

Types of analogy

Analogies can be classified according to their internal logical functioning, as follows:

• Symmetric analogy. Those in which the compared referents can be exchanged without altering the relationships between them. That is, in which A, B, C and D are interchangeable because the relationship remains the same.
• Asymmetric analogy. Those in which the compared referents cannot be exchanged, since their order of appearance designates a specific relationship. That is, A is to B, like C to D, and not B is to A, like C to D.
• Cause and effect analogy. Also known as association analogies, they suppose a specific link, of causality, between the referents. That is, A causes B, as C causes D.
• Analogy by reciprocity. It implies, in the relationship between the referents, a strict and reciprocal need between them, that is to say that for one to exist, the other must exist, reciprocally. That is, A absolutely requires B, as D absolutely requires C.
• Classification analogy. Those that work based on bringing together, in the same set of things, the linked referents. That is, A and B are in the same set, just as B and C are in a similar one.
• Comparative analogy. Those that, when comparing references, seek to highlight a perceivable property. They tend to use links and similes ("as", "such as", "in the same way as", etc.). That is, A is in such a way, as is B.
• Mathematical analogy. Those that exist only between figures, numerical elements and mathematical proportions, given which the referents can have unequal values.

Examples of analogies

The analogy makes it possible to compare feelings of guilt with a heavy burden.

Here are some examples of language analogies:

• Wings are to legs what birds are to people.
• Driver is to car as pilot to plane and machinist to locotomora.
• A painter paints a picture like a poet composes a poem.
• The Sun is he food of the plants.
• The stone is heavy like a guilty conscience.
• The night was dark as was death.
• We are the Spartan warriors of this club reading.
• Mary is the Margaret Thatcher of love.

Argument by analogy

Although so far we have seen analogy as a rhetorical figure, that is, a particular use of language to reach higher expressive levels, it is also true that there is a type of analogical reasoning, which consists of moving from the known to the unknown through of a comparison between referents.

This gives rise to four (main) argumentative forms based on the principle of analogy:

• Interpolation. It is based on the evaluation of all the possible situations of a supposed or imaginary scenario and the repercussions of each one, and then it is transferred to the analyzed situation. For example, suppose a man has two potential lovers, and he refuses to decide on either one. Then a friend advises him and tells him "whoever roasts two rabbits, one gets burned". The imaginary situation then serves to think the real one.
• Extrapolation. This procedure is widely used in the resolution of problems and in teaching, since it is part of the Scientific method. It consists of starting from the assumption that the elements of a scenario will continue to occur in the future, thus allowing us to suppose a new set of rules that would allow us, if true, to reach a new conclusion. For example, suppose a person is undecided about who to vote between two candidates. A friend counsels her and asks her to imagine what would happen if candidate A won, and then what would happen if candidate B won. From both scenarios they extract together conclusions new ones that serve to determine the vote.
• Reduction to the absurd. Its name comes from the Latin Reductio ad absurdum and it serves to demonstrate the validity of categorical propositions. It consists of assuming the hypothetical denial of the validity of the premise, and then obtaining through logical inferences an illogical or fallacious conclusion. For example, suppose a child thinks that Earth it is flat, and his teacher helps him prove that it is not. To do this, he asks you to suppose that the Earth is not round, and how therefore it would be possible to reach the edge, or it would be possible to observe the Sun from any point of the planet. Realizing the absurdity of these logical consequences, the child must accept that the Earth is more likely to be round.
• Modeling. Especially important for the Scientific Method, modeling consists, as its name suggests, in the elaboration of a model hypothetical of the reality, whose results may be analogous to reality, that is, they may be thought in terms analogous to reality. This, for example, occurs with mathematical models of economic behavior, which attempt to predict fluctuations in the world market or in certain currencies.

Analogy in law

In various branches of law analogy plays an important role in arguing for the resolution of a dilemma. In others, however, such as Criminal law, analogies are outlawed by the Principle of legality, which dictates that "there is no crime no shame without law previous".

In any case, the analogy in the law supposes that there must be a similarity between the foreseen and the unforeseen cases, to avoid a radical difference between the two, since the law must be applied without legal loopholes.

This means that, given a past case that was resolved in some way, the same verdict could be applied to a different new case, provided they are sufficiently similar.

Analogy in Biology

In the field of biology and in particular the evolution, is known as an analogy to the superficial similarity between two or more organic structures that nevertheless present different origins. That is, they share essential features but do not come from an immediate common origin.

An example is the wings of a butterfly, a bat and a bird, since in all three cases they serve to fly, but they arose at radically different evolutionary moments. Thus, these organisms all have wings, but for that reason they are not evolutionarily related, nor can they be grouped in that way.