Deductive, inductive, and abductive reasoning are the three most widely used and most useful forms of reasoning. Deductive reasoning involves reasoning from a general rule to a specific conclusion. Inductive reasoning involves developing the most likely general rule from a set of specific observations. Scientific experimentation, which tends to involve observing controlled phenomena to determine rules of physical behavior, is based on inductive reasoning. Abductive reasoning is similar to inductive reasoning, but only involves developing a guess based on what limited data is available at a given time, before detailed testing and rigorous observation.

Deductive reasoning

Deductive reasoning is one of the two basic forms of valid reasoning. It begins with a general hypothesis or known fact and creates a specific conclusion from that generalization. This is the opposite of inductive reasoning, which involves creating broad generalizations from specific observations. The basic idea of deductive reasoning is that if something is true of a class of things in general, this truth applies to all members of that class. One of the keys for sound deductive reasoning, then, is to be able to properly identify members of the class, because incorrect categorizations will result in unsound conclusions.

For deductive reasoning to be sound, the original hypothesis or generalization also must be correct. A logical deduction can be made from any generalization, even if it is not true. If the generalization is wrong, though, the specific conclusion can be logical and valid but still can be incorrect.

One can better understand deductive reasoning by looking at examples. A generalization might be something such as, "All wasps have stingers." The logical conclusion of a specific instance would then be, "That is a wasp, so it has a stinger." This is a valid deduction. The truth of the deduction, however, depends on whether the observed insect is, indeed, a wasp.

People often use deductive reasoning without even knowing it. For example, a parent might say to a child, "Be careful of that wasp — it might sting you." The parent says this because he or she knows that wasps have stingers and, therefore, that the observed wasp has a stinger and might sting the child.

One of the most common and useful forms of deductive reasoning is the syllogism. A syllogism is a specific form of argument that has three easy steps: a major premise, a minor premise and a logical conclusion. For example, the premise "Every X has the characteristic Y" could be followed by the premise "This thing is X," which would yield the conclusion "This thing has the characteristic Y." The first wasp example could be broken up into the major premise "Every wasp has a stinger," the minor premise "This insect is a wasp" and the conclusion "This insect has a stinger." Creating a syllogism is considered a good way for deductive reasoning to be tested to ensure that it is valid.

Inductive Reasoning

Inductive reasoning is a method of drawing a probable conclusion from an emerging configuration of data. In its purest form, this type of reasoning occurs by analyzing unbiased observations and discovering common patterns. When patterns repeat for an extended period of time, an analyst can logically predict that those patterns will continue to repeat. This inference, commonly known as generalization, can produce scientific deductions so probable that they are widely accepted as fact. Any theory involving generalization, however, can be disproved by one instance of inconsistency.

One form of inductive reasoning is the application of certain circumstances to a likely cause. A simple instance of cause-and-effect inference would be the repeated discovery of dead livestock in an area where coyote tracks are also present. While it is theoretically possible that the animals died from natural causes, it is much more likely that their demise was a result of a coyote’s actions.

In medicine, this type of inductive reasoning can be a very powerful diagnostic tool. As a specific illness often presents with a particular list of symptoms, it is reasonable to presume that a patient who exhibits those indicators also has that malady. Most physicians acknowledge that these types of conclusions may be wrong in some instances. In emergency medicine, however, many more lives can be saved by treating the probable condition than are lost by misdiagnosis.

Often, future behavior may be reasonably predicted by inductive reasoning. Logic says that an object that has always behaved in a certain way will continue to behave as such. To simplify Isaac Newton’s work, barring interference, an apple that detaches from a tree will always fall to the ground.

While inductive reasoning of this sort is natural, it is inherently flawed. For example, every day in the history of humanity, the sun has risen, and it can be safely assumed that it will rise tomorrow as well. Scientific evidence, however, shows that the life of a star is long but not unlimited. As such, there will likely come a day when the sun does not rise. In essence, the rising of the sun is not only just a theory but one that is liable to be disproved.

In many ways, the fallibility of inductive reasoning actually increases its strength as a scientific method. Sweeping statements often encourage thorough testing. It is likely, then, that any theory based on inductive reasoning will be challenged repeatedly. Those that survive can be expected to be so accurate that they may be acknowledged as truth.

Abductive Reasoning

Abductive reasoning is a form of reasoning based on the formation and evaluation of hypotheses using the best available information. In many cases, it is synonymous with "educated guessing," the process of guessing based on a reasoned analysis of available information. Abductive reasoning starts with the observation of a phenomenon for which one does not have an immediate, clear explanation. One can then use this form of reasoning to develop an explanation that is sufficient to describe the observed phenomenon, though it must be noted that, without further testing, this explanation is only sufficient, not necessarily accurate. Abductive reasoning is useful in developing hypotheses to be tested, but it is also used for various purposes in artificial intelligence, philosophy, and a variety of other fields.

Many of the most important applications of abductive reasoning are in the day-to-day decisions that almost all people need to make. Most people do not have the time or energy to embark on a detailed scientific investigation before making a given decision, so they use their available knowledge to choose the best course based on educated guesses. Jurors, for example, use such reasoning when making decisions in court, as they must rely on the best available evidence, which is usually not enough to be considered scientifically sufficient for conclusive judgment. Even medical professionals use this form of educated guesswork when making decisions based on diagnostic testing results.

Scientists commonly use abductive reasoning to develop hypotheses to test. A cell biologist who witnesses an interesting change in an organism's eye color after widespread genetic mutation may, for instance, use his available knowledge to develop a guess about which gene is responsible for the color change. Instead of random genetic experiments, he can then focus his work on the gene he suspects is the most relevant. Without abductive reasoning, on the other hand, he would likely not even have this limited guidance.