Elementary, My Dear Watson*

When you’re trying to solve a complex problem, determine a course of action, or evaluate others’ conclusions, you’ll need to engage logical System 2 reasoning, which is the opposite of System 1’s quick assessments.

I never guess. It is a shocking habit—destructive to the logical faculty. —Sherlock Holmes in The Sign of Four

It can be helpful to understand different types of reasoning, be able to identify the type—or types—of reasoning that are being applied in a given situation, and know how accurate each type is likely to be.

But recognizing and/or applying a reasoning process to your problem or evaluation process isn’t enough to guarantee that the outcome of that reasoning process will be sound or accurate. Skillful reasoning doesn’t compensate for faulty premises or missing or biased information.

The following descriptions (but not the examples) of deductive, inductive, and abductive reasoning were provided by Alina Bradford, writing in Live Science (livescience.com).

Deductive reasoning: conclusion guaranteed

Deductive reasoning is a basic form of valid reasoning. Deductive reasoning, or deduction, starts out with a general statement, or hypothesis, and examines the possibilities  to reach a specific, logical conclusion, according to the University of California. The scientific method uses deduction to test hypotheses and theories. “In deductive inference, we hold a theory and based on it we make a prediction of its consequences. That is, we predict what the observations should be if the theory were correct.  We go from the general—the theory—to the specific—the observations,” said Dr. Sylvia Wassertheil-Smoller, a researcher and professor emerita at Albert Einstein College of Medicine.

In deductive reasoning, if something is true of a class of things in general, it is also true for all members of that class. For example, “All men are mortal. Harold is a man. Therefore, Harold is mortal.” For deductive reasoning to be sound, the hypothesis must be correct. It is assumed that the premises, “All men are mortal” and “Harold is a man” are true. Therefore, the conclusion is logical and true.

Examples:

  • It is dangerous to drive on icy streets. The streets are icy now so it is dangerous to drive now.
  • All birds have feathers and robins are birds, so robins have feathers.
  • Elephants have cells in their bodies and all cells have DNA, so elephants have DNA.

[Caveat: Deductive inference conclusions are certain provided the premises are true. It’s possible to come to a logical conclusion even if the generalization is not true. If the generalization is wrong, the conclusion may be logical, but it may also be untrue. For example, the argument, “All bald men are grandfathers. Harold is bald. Therefore, Harold is a grandfather,” is valid logically but it is untrue because the original statement is false.]

Inductive reasoning: conclusion merely likely

Inductive reasoning is the opposite of deductive reasoning. Inductive reasoning makes broad generalizations from specific observations. “In inductive inference, we go from the specific to the general. We make many observations, discern a pattern, make a generalization, and infer an explanation or a theory,” Wassertheil-Smoller told Live Science. “In science there is a constant interplay between inductive inference (based on observations) and deductive inference (based on theory), until we get closer and closer to the ‘truth,’ which we can only approach but not ascertain with complete certainty.”

Even if all of the premises are true in a statement, inductive reasoning allows for the conclusion to be false. Here’s an example: “Harold is a grandfather. Harold is bald. Therefore, all grandfathers are bald.” The conclusion does not follow logically from the statements.

Examples:

  • John is a financial analyst. Individuals with professions in finance are very serious people. John is a very serious person.
  • Jennifer leaves for school at 7:00 a.m. and is on time. Jennifer assumes, then, that she will always be on time if she leaves at 7:00 a.m.
  • The water at the beach has always been about 75 degrees in July. It is July. The water will be about 75 degrees.
Abductive reasoning: taking your best shot

Another form of scientific reasoning that doesn’t fit in with inductive or deductive reasoning is abductive. Abductive reasoning usually starts with an incomplete set of observations and proceeds to the likeliest possible explanation for the group of observations (Critical Thinking Skills, Butte College). It is based on making and testing hypotheses using the best information available. It often entails making an educated guess after observing a phenomenon for which there is no clear explanation.

Abductive reasoning is useful for forming hypotheses to be tested. Abductive reasoning is often used by doctors who make a diagnosis based on test results and by jurors who make decisions based on the evidence presented to them.

Examples:

  • Given a particular set of symptoms, a medical doctor needs to determine the diagnosis that would best explain most of them.
  • Jurors have to decide whether the prosecution or the defense has the best explanation to cover all the points of evidence although additional evidence may exist that was not admitted in the case.

While using one of these three types of reasoning is a function of System 2 (conscious) cognition, evaluating them—and their results—is an example of metacognition, which is a higher order of System 2 cognition. Metacognition is a skill you can develop to help you think smarter and improve outcomes in all areas of your life.

I cannot live without brain-work. What else is there to live for? —Sherlock Holmes in The Sign of Four


*This quintessential Sherlock Holmes quote was never actually uttered in any of Conan Doyle’s stories about him.

Why Right-Brain Left-Brain Is Wrong-Headed

left brain right brain

Conventional wisdom has it that some people are right-brained, meaning they tend to be creative, intuitive, and emotional, while others are left-brained, meaning they tend to be logical, analytical, and methodical. But once again conventional wisdom has vastly oversimplified and overstated the situation. (There’s a reason why conventional wisdom tends to function this way, but that’s another blog post.)

It’s true that the two hemispheres of the brain function differently. Much of what we know about the differences between the two hemispheres is the result of research conducted in the 1960s on patients with split brains. Normally the two hemispheres are in ongoing communication with each other via the bridge of fibers called the corpus callosum. But the corpus callosum was surgically severed in some epilepsy patients in a last-ditch attempt to relieve their symptoms.

Michael Gazzaniga, Roger Sperry, and two other researchers conducted the testing on these individuals that revealed some of the effects—many of them quite surprising—of cutting off communication between the hemispheres. Later on, Gazzaniga conducted additional research with Joseph LeDoux.

To me, the most interesting thing they discovered is what happens as a result of visual information no longer being passed from one hemisphere to another. Language is primarily a function of the left hemisphere. So although the right hemisphere could recognize an image not shown to the left hemisphere, it couldn’t communicate about it verbally. David Eagleman summarizes these experiments in his book Incognito. Remember that brain wiring is contralateral, which means that the right hemisphere processes information from the left visual field and controls the movements of the left hand—and vice versa for the left hemisphere.

In 1978, researchers Michael Gazzaniga and Joseph LeDoux flashed a picture of a chicken claw to the left hemisphere of a split-brain patient and a picture of a snowy winter scene to his right hemisphere. The patient was then asked to point at cards that represented what he had just seen. His right hand pointed to a card with a chicken, and his left hand pointed to a card with a snow shovel.

The experimenters asked him why he was pointing to the shovel. Recall that his left hemisphere (the one with the capacity for language), had information only about a chicken, and nothing else. But the left hemisphere, without missing a beat, fabricated a story: “Oh, that’s simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.”

When one part of the brain makes a choice, other parts can quickly invent a story to explain why. If you show the command “Walk” to the right hemisphere (the one without language), the patient will get up and start walking. If you stop him and ask why he’s leaving, his left hemisphere, cooking up an answer, will say something like “I was going to get a drink of water.”

The chicken/shovel experiment led Gazzaniga and LeDoux to conclude that the left hemisphere acts as an “interpreter,” watching the actions and behaviors of the body and assigning a coherent narrative to these events. And the left hemisphere works this way even in normal, intact brains.

One important thing to remember is that for people with intact brains, the two hemispheres remain in constant communication with each other. We are whole-brained people who use both parts of our brain all the time, including during the creative process and in the course of logical problem-solving.

Although this, too, is an oversimplification, it’s closer to the mark to say that if we did not have language or discernment, our creative ideas would be useless and possibly incoherent. And if we did not have emotion and imagination, we would have no context for decision making.

Not only is neither hemisphere “better” than the other, you may be surprised at the conclusion Gazzaniga has reached about which hemisphere is more “conscious” and which hemisphere is more literal.

After many years of fascinating research on the split brain, it appears that the inventing and interpreting left hemisphere has a conscious experience very different from that of the truthful, literal right brain. Although both hemispheres can be viewed as conscious, the left brain’s consciousness far surpasses that of the right. Which raises another set of questions that should keep us busy for the next 30 years or so.

I’m looking forward to the results of that additional research!