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.

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