Reasoning by Analogy

“Withdrawal of U.S. troops will become like salted peanuts to the American public; the more U.S. troops come home, the more will be demanded.” – Henry Kissinger in a Memo to President Richard Nixon.

This striking example of reasoning by analogy argues against troop withdrawal by linking it to a common human feeling when eating salted peanuts. How and why might we be swayed by analogical reasoning (the formal term for reasoning by analogy)?

The bare structure of the reasoning has three components.

  • Source idea: Eating salted peanuts awakens a desire. People want another.
  • Implied general trait: The initial satisfaction makes the second request more likely.
  • Apply to target idea: The first troop withdraw will awaken the desire and people will want another troop withdrawal.

Analogical Interpretation

A more general structure of this analogical reasoning is:

  • Premise 1: Source A has traits {1, 2} among others. Source A leads to result 3.
  • Premise 2: Target B shares traits {1, 2}.
  • Conclusion: Therefore, Target B leads to result 3.

The argument form is inductive, also called ampliative. Since the conclusion is a generalization, it is not guaranteed to be true even if the premises are.

Thus, reliance on reasoning by analogy requires careful evaluation. It’s necessary to ensure that the similarities are relevant and sufficient, and that the differences we ignore are not important or significant.

How does analogical reasoning arise in the brain? What is the neurological basis for associating results from similar, but not exact, situations?

Neural Mechanism

Our neural brain automatically groups inputs of our senses into categories with shared features. This occurs because of the combination of the neuron’s All-or-None firing and Hebb’s observation of neurons that fire together, wire together. Those two features allow the grouping of similar inputs into the same category. A side effect is they ignore any detail beyond that necessary to fire the neuron.

Let’s consider the processing steps of sensory data in Figure 1. Neural Impact Stages.

Figure 1 Neural Impact Stages. Processing sensory data until it reaches conscious awareness.

The left column in Figure 1 lists the sequence of neural events that occur with an event.

  • Sensation recognition relies on the All-or-None firing (sending signals) from one neuron to other neurons.
  • Integration of sensory information depends on the regularity that neurons that fire together wire together.
  • Information is passed along neural pathways towards our prefrontal lobes for processing. These are more integration steps which depending on neurons that fire together, wire together.
  • Augmented sensory information arrives in the executive areas of the prefrontal lobes.
  • Decision-making requires reasoning based on logic and analogies.

The next column to the left, Neurons, lists the neural properties used on the row.

  • The neural threshold property of neurons governs whether the neuron sends a signal. If the sum of its inputs is less than the threshold, then no signal is sent. If the sum is equal to or greater than the threshold, then the neuron sends a signal through its axon. Importantly, the same signal whether the threshold is barely exceeded or greatly exceeded.
  • Hebb’s theory states that repeated firing of the same neurons leads to those same neurons to fire in unison in the future.
  • Hebb’s theory is not restricted to sensory neurons. It applies across all the neural pathways in the brain.
  • Neurons in working memory have developed specialized, collaborative features. Besides handling verbal categories, working memory handles individualized patterns and can exchange those words and patterns.
  • Some neural modules have specialized their inherent associations through categories to handle verbal categories with logic.

The Implications column mentions the main impact of the neural property active on each stage.

  • Loss of details references the All-or-None signal property of transmitting only that a neuron fired. Information about which inputs to the neuron caused it to signal is lost. Downstream neurons do not know the details of upstream activity, only the result.
  • Losing detail implies that multiple input sensations result in the same signal. The group of all these inputs forms a category.
  • As the signals travel the neural pathways, they are passed forward when neural thresholds are exceeded. As the signals continue towards the prefrontal executive areas, further categorization takes place as memories, knowledge, and 3S biological imperatives (Satiety, Sex, and Safety) are merged with the original sensory data.
  • Sensory inputs have multiple categories by the time they arrive in our consciousness. Multiple categories are available interpretations (connotations) of the impact of the sensory event on us.
  • Rational considerations of our reaction to an event can be iterative, switching between various abstract categories, searching for the most optimal response.

The last column, Psychology, lists common terms that capture the stage.

  • Abstraction. The loss of detail.
  • 1st Category. Abstraction leads to multiple sensory experiences sharing most features to be cast into the same category.
  • 2nd Category. Integrating a category along neural pathways leads to a second category of higher abstraction and further additional categories of higher abstraction (more generality).
  • Association. Any memory within the same category is available.
  • Substitution. To determine if it offers a better response to the situation, we consider another memory, with different details.

With that background, we can discuss how a salted peanuts observation leaped to troop withdraw caution in Figure 2. Kissinger’s Source and Target Analogy.

Figure 2. Kissinger's Source and Target Analogy. Consider by neural impact stages.

The leftmost column in Figure 2 identifies the neural impact on the Process column that specifies the processing performed. The third column displays the content of the analogy source. The rightmost column displays the target content.

Analogy Source. Salted Peanuts

From the first row, we see that abstraction occurs in sensory processing, resulting in awareness that salted peanuts are tasted.

In the second row, the person forms category 1 by placing the taste based on their prior experiences.

In the third row, an assignment to a further abstracted category—such as favorite level of salt, or a special fondness due to a relative who always had salted peanuts, or merely a substance that would quiet the rumbling of a hungry stomach. There are multiple steps as the sensation continues its path to conscious awareness. This results in multiple categories associated with the sensation.

In the fourth row, we are aware of the sensation with its various categories. For the analogy source, Kissinger has selected the desire category.

In the bottom most row, cause-and-effect decision-making led Kissinger to conclude that the eating the salted peanut will cause a person to want another salted peanut. He ignored any evidence to the contrary that would clutter his point.

Analogy Target. Withdrawal Impact

In Figure 2, the first three rows in the fourth column are blank. The analogy starts at a higher level of abstraction than actual physical reality.

In the fourth row, in a person’s conscious awareness, Kissinger claimed a direct link between salted peanuts and troop withdrawal, although they differ in many details.

In the last row, during decision-making, the analogy asserts the consequence of withdrawing troops will be a desire for more troops to be withdrawn, because of the analogical relationship.

Considerations for Relying Reasoning by Analogy

The three missing entries in the fourth column of Figure 2 reveal the large scope of potential differences ignored when asserting the analogy.

Several crucial rules, courtesy Stanford Encyclopedia of Philosophy, for evaluating analogical reasoning are:

  • The more similarities (between two domains), the stronger the analogy.
  • The more differences, the weaker the analogy.
  • The greater the extent of our ignorance about the two domains, the weaker the analogy.

Widespread Use and Reliance on Reasoning by Analogy

Reasoning by analogy is bolstered by three neural features:  neural threshold for sending a signal, the All-or-None nature of signals, and Hebb’s Law that neurons that fire-together, wire-together. However, neural support does not guarantee the truth or utility of the target conclusion, because the ignored differences may outweigh superficial similarities.

So, if the conclusion may be wrong, why do we ever use reasoning by analogy?

Because we often don’t have enough facts to complete a deductive argument. In Kissinger’s quote to Nixon, we saw that reasoning by analogy allowed Kissinger to jump over missing information to make his caution about troop withdrawal.

Mountain scape

Galileo’s analogy between the visual effects of mountains on Earth and his telescopic observations on the Moon’s terminator led him to declare there were mountains on the Moon in 1609, more than three and a half centuries before we visited it.


Citation. Mountains on the Moon image created by Bing at my prompt.

Additional Information
Stanford Encyclopedia of Philosophy article on Analogy and Analogical Reasoning.

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