One of the most important skills for a person to learn—whether they are a student, professional academic, or any other informed citizen—is the ability to think logically. More than anything else, this skill allows a person to effectively evaluate whether information is credible and whether an idea is sound. Without strong critical thinking skills, people become vulnerable to all sorts of false information—from dishonest advertising, to unhealthy peer pressure, to dangerous conspiracy theories. Strong critical thinking skills allow people to find the most plausible explanation for observations, find definitive solutions when possible, identify concepts that are not strongly supported by evidence and need further evaluation, and generate new hypotheses worth testing. Critical thinking is what allows humans to add to our collective knowledge.
There are three types of reasoning that contribute to academic progress: "deduction", "induction", and "abduction."
- Deduction is a general-to-specific form of reasoning that allows one to apply known truths to specific instances with a high degree of certainty.
- Induction is a specific-to-general form of reasoning that tries to generate broad rules that can be applied in many circumstances.
- Abduction is a specific-to-general form of reasoning that specifically looks at cause and effect, often for a particular example.
Let's consider each one in turn.
Deductive reasoning
Deductive reasoning uses generally accepted statements or facts to arrive at a logical conclusion for a specific question. Math is deductive.
- If x = 10
- And if y = 7
- Then x + 2y = 24
Deductive reasoning often takes the form of a syllogism. A major premise and a minor premise are used to reach a logical conclusion, which is also called an inference or deduction.
- Premise 1: Every A is B.
- Premise 2: This C is A.
- Inference: This C is B.
- Premise 1: All birds have feathers.
- Premise 2: Penguins are birds.
- Inference: Penguins have feathers.
If premise 1 and premise 2 are true, then the inference must be true.
The reliability of the inference depends on the strength of the premises.
- Premise 1: All birds can fly.
- Premise 2: Penguins are birds.
- Inference: Penguins can fly.
Here, the initial premise is incorrect, which makes the inference incorrect. However, there was a time when people thought that all birds could fly (so premise 1 was assumed to be correct). Then, the discovery of animals that shared so many characteristics with birds (feathers, warm-blooded, hard-shelled eggs, beak, etc.) caused scientists to reconsider the definition of a bird. Rather than creating separate categories for the many bird-like creatures that cannot fly (penguins, ostriches, kiwis, etc), they found that the simplest strategy was to allow for birds that had lost the ability to fly.
Deductive reasoning is used to refine ideas
The example above shows how deductive reasoning can be used to test the reliability of generally accepted facts, and of new ideas under consideration. Inconsistencies require ideas or definitions to be refined, so that they are more accurate.
- Premise 1: All birds lay eggs.
- Premise 2: Male penguins are birds.
- Inference: Male penguins lay eggs.
While premise 1 and premise 2 are generally accepted as true, the inference is incorrect. The major premise (premise 1) needs to be revised. One possibility is as follows.
- Premise 1: All female birds lay eggs.
- Premise 2: Penguins are birds.
- Inference: Female penguins lay eggs.
Note that a premise can be false without making the inference false.
- Premise 1: All birds can fly.
- Premise 2: Eagles are birds.
- Inference: Eagles can fly.
While the inference is correct (at least for healthy adult eagles), this is still an example of poor reasoning because premise 1 is not true.
Incorrect use of deductive reasoning
It's also possible for both premises to be true, but for the inference to be incorrect because the major premise doesn't exclude other possibilities:
- Premise 1: Some birds can fly.
- Premise 2: Penguins are birds.
- Inference: Penguins can fly.
Furthermore, an inference can be incorrect because the two premises are unrelated.
- Premise 1: Birds can fly.
- Premise 2: Moths can fly.
- Inference: Moths are birds.
Incorrect use of deductive reasoning is a common ploy to spread false information.
- Premise 1: People trying to reduce the spread of COVID-19 often stand 6 feet apart.
- Premise 2: Satanists often stand 6 feet apart.
- Inference: People trying to reduce the spread of COVID-19 are Satanists.
None of these combinations meet the stringent requirements for a syllogism, which are:
- Premise 1: Every A is B.
- Premise 2: This C is A.
- Inference: This C is B.
In deductive reasoning, the strength of an inference depends on the strength of the 2 premises, and their relevance to each other.
Deductive reasoning in everyday life
Deductive thinking is often used to make everyday decisions, when the connections are quite straightforward.
- Premise 1: It takes 30 minutes to drive to the doctor's office from home.
- Premise 2: My doctor's appointment is at 9:00am.
- Inference: I should leave the house by 8:30am to arrive at the doctor's office on time.
- Premise 1: In sociology research, there are often significant differences based on age and/or gender.
- Premise 2: I will be conducting a sociology study.
- Inference: My sociology research should include information on each participant's age and gender.
Inductive reasoning
Inductive reasoning starts with observations that are limited in scope, and proceeds to a more generalized conclusion that may be true. Confidence in the validity of a conclusion can vary widely, depending on the quality and number of observations that support it.
The example below inspires high confidence.
- Observation 1: The sun rose this morning.
- Observation 2: The sun has risen every morning so far.
- Generalization: The sun rises every morning.
Supported by enough evidence, generalizations made through inductive reasoning become the "generally accepted facts" that are used in deductive reasoning.
Here is another example:
- Observation 1: In the 1980s, young gay men who were previously healthy started dying from severe immune deficiency, a condition which was named acquired immune deficiency syndrome (AIDS).
- Observation 2: The blood of these patients was found to contain a novel RNA virus, named human immunodeficiency virus (HIV).
- Generalization: HIV is the virus that causes AIDS.
There were other possible explanations. For example, while HIV is present in these patients, their immune deficiency could be caused by something else. This alternative explanation was consistent with the observation that some people carry HIV without developing AIDS. In the end, the hypothesis that HIV is the virus that causes AIDS turned out to be correct, but the possible alternative explanations helped guide research. This led to a better understanding of the disease, the development of "generally accepted facts" about HIV, and eventually to effective treatments.
In science, there is a constant interplay between inductive reasoning to generate hypotheses, and deductive reasoning to test the validity of hypotheses.
Inductive reasoning often involves probability, and can be used in everyday life.
- Observation: On any given day from October through May, there is a 50% chance of rain in Seattle.
- Generalization: If you're visiting Seattle anytime from October through May, it is a good idea to bring an umbrella.
Abductive reasoning
The difference between abductive reasoning and inductive reasoning is subtle, and not universally agreed upon. Both use evidence to determine what is likely—but not guaranteed—to be true. Abductive reasoning looks for cause-and-effect relationships, while induction seeks to determine general rules. Both work with the information that is available, which is usually incomplete. A criminal trial is an application of abductive reasoning. The jury must decide whether the prosecution or the defense provides the most likely explanation for all the evidence.
A medical diagnosis is another example of abductive reasoning. Given a patient's symptoms and medical history, what diagnosis would best explain their situation? A diagnosis is essentially a hypothesis, which can often be tested by taking blood samples, ordering X-rays, trying medications, etc. It's common for patient information to be incomplete or inaccurate in some ways, and for a diagnosis to not explain all symptoms.
Let's consider this example.
- Observation 1: Patient is experiencing fatigue, fever, and unintentional weight loss.
- Observation 2: Patient has a history of unprotected sex with numerous partners.
- Possible cause of symptoms: HIV infection
While there are many possible explanations for this patient's symptoms, observation 2 is a significant risk factor for HIV infection. Any prudent doctor would have the patient tested for HIV, in addition to the other common causes of their symptoms. The doctor would then apply a combination of deductive and inductive reasoning to reach a diagnosis.
Abduction can lead to some groundbreaking hypotheses.
- Observation 1: A patient has been HIV positive for over 10 years, but has not developed AIDS.
- Observation 2: This patient has never taken any anti-HIV medications.
- Possible cause: This patient has an inborn, genetic resistance to HIV.
This possible explanation begs to be tested, by comparing DNA sequences from this patient and similar patients to DNA sequences from the general population. Inductive reasoning would be applied to generate broad rules that could apply to many circumstances, and those rules would be tested for accuracy with deductive reasoning.
A combination of all three types of reasoning—deductive, inductive, and abductive—is used to expand human knowledge.