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Analogy and Scientific Thought

May 9, 2016

According to a now discredited notion, the operating mode of scientists is fixed by whether they are right-brained, or left-brained. Early scientific studies seemed to indicate that the brain's right hemisphere is the seat of intuition and creativity, while the brain's left hemisphere is focused on logic and problem solving. Thus, according to Kuhn's dichotomy, the left-brained scientists practice "normal" science, while the right-brained scientists are the paradigm shifters.

This concept of functional brain asymmetry began with observations that damage to the left side of the brain caused difficulty in understanding language, while damage to the right side of the brain impaired spatial ability.

Modern brain studies that now use functional neuroimaging techniques have shown that this view of brain function is too simplistic. All parts of the brain "light-up" in different thought processes, although some parts more than others. What still remains valid is that the right side of the body is controlled by the left hemisphere, and vice versa.[1]

Functional magnetic resonance image

A functional magnetic resonance image showing brain regions activated when viewing a moving visual stimulus.

(Image by Washington irving, via Wikimedia Commons.)

When reviewing the history of science, it's apparent that many scientific theories were based on analogy with the observed properties of other systems. As Ralph Waldo Emerson said in his 1837 speech, The American Scholar, "... science is nothing but the finding of analogy, identity, in the most remote parts."[2] The Bohr model of the atom resembles a miniature solar system for the simple reason that physicists at that time believed that both systems involved bodies under the influence of the same inverse-square law attractive force.

Another example is the analogy between electrical circuits and hydraulics, as shown in the figure. It's likely that the hydraulic analogy helped to conceptualize the development of some important circuit theory concepts, such as Kirchoff's law. Now, the analogy remains as a teaching aid.

The hydraulic analogy of electrical circuits

The hydraulic analogy of electrical circuits. In this analogy, voltage is associated with pressure, current with fluid flow, and resistance with the flow restriction caused by small diameter pipes. (Created using Inkscape.)

When a biologist tests a drug or potentially toxic chemical on a mouse with the idea that he will obtain information relevant to the affect of that substance on a human, he's performing what philosophers call an argument from analogy. Since mice survive by essentially the same physiological processes as humans, it's expected that there will also be a commonality in the organism's reaction to this new substance. Unfortunately, there are sometimes problems with this approach, as shown by phocomelia associated with thalidomide.

Mathematics allows us to quantify observations, but mathematical concepts are used also as analogy to guide research. There's no real reason, as far as we know, why a mathematical prediction should lead to an observation, but it often does. For example, a certain symmetry of subatomic particles caused physicists to believe that a missing particle, the Omega-minus (Ω-) particle, should exist, and it was discovered in 1964 (see figure).

Baryon Decuplet

A baryon decuplet of Delta (Δ), Sigma (Σ), Xi (Ξ), and Omega (Ω) baryons.

In this illustration, Q is electric charge and S is strangeness.

(Modified Wikimedia Commons image.)

Johannes Kepler, best known for deducing the laws of planetary motion, was an ardent practitioner of analogy who's quoted as saying, "...I especially love analogies, my most faithful masters, acquainted with all the secrets of nature."[3] In his Harmonices Mundi, Kepler attempted to justify an ancient idea, called musica universalis, that expresses an analogy between the harmony of planetary movement and musical harmony.

A figure on page 47 of Book III of Kepler's Harmonices Mundi

A figure on page 47 of Book III of Johannes Kepler's Harmonices Mundi. (PDF text via Wikimedia Commons.[4]

Kepler also explained the motion of the planets by analogy to the motion of a ship in a current of water, with the Sun generating some sort of circular river.[5] He had a further magnetism analogy in which the Earth was like a lump of iron influenced by a magnetic Sun.[5] He was surprisingly close to the mark by having an iron Earth.

Sadi Carnot, in his Reflections on the Motive Power of Fire, made an analogy between water falling through a waterfall and heat, his caloric fluid, "falling" through a heat engine.[3] Robert Boyle, known especially for Boyle's law, justified his idea that many small particles can combine to have a large affect with an analogy to the ability of a swarm of ants to move large objects.[3]

Sadi Carnot (left) and Robert Boyle (right)

Sadi Carnot (left) and Robert Boyle (right)

(Carnot image, and Boyle image, via Wikimedia Commons.)

Alchemy, the root of modern chemistry, had the strange analogy that the alloying of two component metals was like a marriage. The alchemists ascribed maleness to such elements as mercury, and femaleness to sulfur.[3] Psychologist and philosopher, William James, thought that reasoning by analogy was a part of human nature. As he wrote in his 1890 book, The Principles of Psychology, "Men, taken historically, reason by analogy long before they have learned to reason by abstract characters."[6]

William James may just have been proven correct more than a century after he penned this conjecture. Scientists from Carnegie-Mellon University (CMU) have just shown that the concept of analogy in science runs far deeper than our use of one scientific observation to describe another. Such analogy in the service of science appears to be hardwired into the brain.[7]

Robert Mason and Marcel Just of CMU used functional magnetic resonance imaging on nine advanced physics and engineering student to show that the brain centers used for such early human tasks such as hunting, foraging, and survival, are re-purposed to understand such abstract concepts as momentum, energy and gravity. More specifically, the brain is able to learn physics concepts because of its ability to understand the four fundamental concepts of causal motion, periodicity, energy flow, and sentences describing quantities.

They suggest that this finding might be useful in science education, since it suggests to teachers better ways to teach these abstract scientific concepts. Says Mason, a senior research associate in CMU's Department of Psychology,
"If science teachers know how the brain is going to encode a new science concept, then they can define and elaborate that concept in ways that match the encoding. They can teach to the brain by using the brain's language."

In the experiments, the brain of each student was scanned while they were shown a set of thirty concepts familiar to scientists and engineers, such as inertia, velocity, gravity, entropy, and refraction. It was found, for example, that the brain regions excited while hearing the steady gallop of a horse were the same that were excited while contemplating the mechanics of waves. Likewise regions that would have been excited when Newton saw his apple fall are the ones involved with the abstract physical concept of gravity.

The concept of energy flow was found to have a commonality to sensing warmth from the Sun or other heat source. Perusing mathematical equations evokes the same brain systems used to comprehend sentences that described quantities. Says co-author of the study and CMU professor, Marcel Just,
"This is why humans have been able to move ahead and innovate -- because we can use our brain for new purposes... Human brains haven't changed much over a few thousand years, but new fields like aeronautics, genetics, medicine and computer science have been developed and continuously change. Our findings explain how the brain is able to learn and discover new types of concepts."

Functional magnetic resonance imaging of brain regions activated by certain concepts in physics

Functional magnetic resonance imaging of brain regions activated by certain concepts in physics. The brain is able to learn physics concepts since they relate to the everyday ideas of causal motion, periodicity, energy flow and sentence-like structure. This research was funded by the Office of Naval Research. (Carnegie Mellon University image.)


  1. Tania Lombrozo, "The Truth About The Left Brain / Right Brain Relationship," NPR, December 2, 2013.
  2. Ralph Waldo Emerson, "The American Scholar," 1837, via Hanover College Web Site.
  3. Dedre Gentner and Michael Jeziorski, "The shift from metaphor to analogy in Western science," From Metaphor and Thought, Andrew Ortony, Ed. (Cambridge University Press, 1993), ISBN:9780521405614, pp. 447-480 (2 MB PDF File).
  4. Johannes Kepler, "Harmonices Mundi," 1619, Latin Text via Archive.org (24 MB PDF file).
  5. Dedre Gentner, Sarah Brem, Ron Ferguson, Philip Wolff, Arthur B. Markman, and Ken Forbus, "Analogy and creativity in the works of Johannes Kepler,"In T. B. Ward, S. M. Smith, and J. Vaid, Eds., "Creative thought: An investigation of conceptual structures and processes," American Psychological Association (Washington, DC, 1997), pp. 403-459.
  6. William James, "The Principles of Psychology," Vol. II, Henry Holt and Company (New York, 1905), p. 363, via Archive.org.
  7. Scientists discover how the brain repurposes itself to learn scientific concepts, Carnegie Mellon University Press Release, April 12, 2016. A preprint of this study is available here (0.5 MB PDF file).

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