An Analogy is a relation of similarity between two or more things, so that an inference (reasoning from premise to conclusion) is drawn on the basis of that similarity. So if item or person or process A is known to have certain characteristics, and if item or person or process B is known to have at least some of those characteristics, the inference is drawn that B also has those other characteristics. If the cases are not similar enough to warrant the inference, then it is a false analogy.
An analogy is either the cognitive process of transferring information from a particular subject (the analogue or source) to another particular subject (the target), or a linguistic expression corresponding to such a process. In a narrower sense, an analogy is an inference or an argument from a particular to another particular, The word analogy can also refer to the relation between the source and the target themselves, which is often, though not necessarily, a similarity, as in the biological notion of analogy.
Analogy has been studied and discussed since classical antiquity by philosophers, scientists, and lawyers. The last few decades have shown a renewed interest in analogy, most notable in cognitive science.
With respect to the terms source and target, there are two distinct traditions of usage:
Analogy plays a significant role in problem solving, decision making, perception, memory, creativity, emotion, explanation and communication. It lies behind basic tasks such as the identification of places, objects and people, for example, in face perception and facial recognition systems. It has been argued that analogy is "the core of cognition" (Gentner et al. 2001). Specifically analogical language comprises exemplification, comparisons, metaphors, similes, allegories, and parables, but not metonymy. Phrases like "and so on," "and the like," "as if," and the very word "like" also rely on an analogical understanding by the receiver of a message including them. Analogy is important not only in ordinary language and common sense, where proverbs and idioms give many examples of its application, but also in science, philosophy and the humanities. The concepts of association, comparison, correspondence, homomorphism, iconicity, isomorphism, mathematical homology, metaphor, morphological homology, resemblance, and similarity are closely related to analogy. In cognitive linguistics, the notion of "conceptual metaphor" may be equivalent to that of analogy.
In ancient Greek the word αναλογια (analogia) originally meant proportionality, in the mathematical sense, and it was indeed sometimes translated to Latin as proportio. From there analogy was understood as identity of relation between any two ordered pairs, whether of mathematical nature or not. Kant's Critique of Judgment held to this notion. Kant argued that there can be exactly the same relation between two completely different objects. The same notion of analogy was used in the U.S.-based SAT tests, that included "analogy questions" in the form "A is to B as C is to what?" For example, "Hand is to palm as foot is to ____?" These questions were usually given in the Aristotelian format:
It is worth noting that while most competent English speakers will immediately give the right answer to the analogy question (sole), it is quite more difficult to identify and describe the exact relation that holds both between hand and palm, and between foot and sole. This relation is not apparent in some lexical definitions of palm and sole, where the former is defined as "the inner surface of the hand," and the latter as "the underside of the foot." Analogy and abstraction are different cognitive processes, and analogy is often an easier one.
Greek philosophers such as Plato and Aristotle actually used a wider notion of analogy. They saw analogy as a shared abstraction (Shelley 2003). Analogous objects shared an idea, a pattern, a regularity, an attribute, an effect or a function. They also accepted that comparisons, metaphors and "images" (allegories) could be used as valid arguments, and sometimes they called them "analogies." Analogies should also make those abstractions easier to understand and give confidence to the ones using them.
The Middle Ages saw an increased use and theorization of analogy. Roman lawyers had already used analogical reasoning and the Greek word analogia. Mediaeval lawyers distinguished analogia legis and analogia iuris. In theology, analogical arguments were accepted in order to explain the attributes of God. Aquinas made a distinction between equivocal, univocal, and analogical terms, the latter being those like healthy that have different but related meanings. Not only a person can be "healthy," but also the food that is good for health (see the contemporary distinction between polysemy and homonymy). Thomas Cajetan wrote an influent treatise on analogy. In all of these cases, the wide Platonic and Aristotelian notion of analogy was preserved.
Some philosophers, especially William of Ockham, rejected any analogy of being—that is, any argument or inference based on the claim that two beings are alike—because they held that the concept of being is univocal.
The most famous theological argument from analogy was given by Bishop William Paley (1743-1805). He argued that if, in walking across a heath, one were to find a watch lying on the ground, one would conclude that the watch had been designed and made by someone—Paley called it an intelligent artificer—and that it had not just appeared there by an unexplainable accident or simple process of nature. By analogy, Paley claimed, the complex structures of living things show an equal or even greater complexity and precision of structure, which means that they must have been made by an intelligent designer, namely God. Paley's is the most well-known and most persuasive theological or religious argument against Darwinian and neo-Darwinian evolution. Evolutionary biologist Richard Dawkins explicitly cites and praises Paley's argument, but then goes on to reject and attempt to refute it in his book, The Blind Watchmaker.
Theologian Karl Barth replaced the analogy of being (analogia entis) with an analogy of faith (analogia fidei) since, he claimed, religious truth (and faith) are God-given.
Against the medieval writers on analogy, Bacon and later Mill argued that analogy be simply a special case of induction (see Shelley 2003). In their view, analogy is an inductive inference from common known attributes to another probable common attribute, which is known only about the source of the analogy, in the following form:
This view does not accept analogy as an autonomous or independent mode of thought or inference, reducing it to induction. However, autonomous analogical arguments are still useful in science, philosophy and the humanities (see below).
The opposite move could also be tried, reducing analogy to deduction. It is argued that every analogical argument is partially superfluous and can be rendered as a deduction stating as a premise a (previously hidden) universal proposition which applied both to the source and the target. In this view, instead of an argument with the form:
One should have:
This would mean that premises referring the source and the analogical relation are themselves superfluous. However, it is not always possible to find a plausibly true universal premise to replace the analogical premises (see Juthe 2005). And analogy is not only an argument, but also a distinct cognitive process.
Contemporary cognitive scientists use a wide notion of analogy, extensionally close to that of Plato and Aristotle, but framed by the structure mapping theory (Dedre Gentner et. al. 2001). The same idea of mapping between source and target is used by conceptual metaphor theorists. Structure mapping theory concerns both psychology and computer science.
According to this view, analogy depends on the mapping or alignment of the elements of source and target. The mapping takes place not only between objects, but also between relations of objects and between relations of relations. The whole mapping yields the assignment of a predicate or a relation to the target.
Structure mapping theory has been applied and has found considerable confirmation in psychology. It has had reasonable success in computer science and artificial intelligence. Some studies extended the approach to specific subjects, such as metaphor and similarity (Gentner et. al. 2001 and Gentner's publication page).
Keith Holyoak and Paul Thagard (1997) developed their multiconstraint theory within structure mapping theory. They defend that the "coherence" of an analogy depends on structural consistency, semantic similarity and purpose. Structural consistency is maximal when the analogy is an isomorphism, although lower levels are admitted. Similarity demands that the mapping connects similar elements and relations of source and target, at any level of abstraction. It is maximal when there are identical relations and when connected elements have many identical attributes. An analogy achieves its purpose insofar as it helps solve the problem at hand. The multiconstraint theory faces some difficulties when there are multiple sources, but these can be overcome (Shelley 2003). Hummel and Holyoak (2005) recast the multiconstraint theory within a neural network architecture.
A problem for the multiconstraint theory arises from its concept of similarity, which, in this respect, is not obviously different from analogy itself. Computer applications demand that there are some identical attributes or relations at some level of abstraction. Human analogy does not, or at least not apparently.
Douglas Hofstadter and his team (see Chalmers et. al 1991) challenged the shared structure theory and mostly its applications in computer science. They argue that there is no line between perception, including high-level perception, and analogical thought. In fact, analogy occurs not only after, but also before and at the same time as high-level perception. In high-level perception, humans make representations selecting relevant information from low-level stimuli. Perception is necessary for analogy, but analogy is also necessary for high-level perception. Chalmers et. al. conclude that analogy is high-level perception. Forbus et. al. (1998) claim that this is only a metaphor. It has been argued (Morrison and Dietrich 1995) that Hofstadter's and Gentner's groups do not defend opposite views, but are instead dealing with different aspects of analogy.
Some types of analogies can have a precise mathematical formulation through the concept of isomorphism.
In anatomy, two anatomical structures are considered to be analogous when they serve similar functions but are not evolutionarily related, such as the legs of vertebrates and the legs of insects. Analogous structures are the result of convergent evolution and should be contrasted with homologous structures.
In law, analogy is used to resolve issues on which there is no previous authority. A distinction has to be made between analogous reasoning from written law and analogy to precedent case law.
In civil law systems, where the preeminent source of law are legal codes and statutes, a lacuna (a gap) arises when a specific issue is not explicitly dealt with in written law. Judges will try to identify a provision whose purpose applies to the case at hand. That process can reach a high degree of sophistication, as judges sometimes not only look at specific provision to fill lacunae (gaps), but at several provisions (from which an underlying purpose can be inferred) or at general principles of the law to identify the legislator's value judgement from which the analogy is drawn. Besides the not very frequent filling of lacunae, analogy is very commonly used between different provisions in order to achieve substantial coherence. Analogy from previous judicial decisions is also common, although these decisions are not binding authorities.
By contrast, in common law systems, where precedent cases are the primary source of law, analogies to codes and statutes are rare (since those are not seen as a coherent system, but as incursions into the common law). Analogies are thus usually drawn from precedent cases: The judge finds that the facts of another case are similar to the one at hand to an extent that the analogous application of the rule established in the previous case is justified.
Often a physical prototype is built to model and represent some other physical object. For example, wind tunnels are used to test scale models of wings and aircraft, which act as an analog to full-size wings and aircraft.
For example, the MONIAC (an analog computer) used the flow of water in its pipes as an analog to the flow of money in an economy.
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