Elementary, My Dear Watson

While mystery readers recognize and appreciate “Holmesian deduction,” all students of logic would do well to be familiar with three important forms of reasoning or argument.

Remember, any argument consists of two essential parts: (1) a claim or conclusion; and (2) support (premises) for the claim in the form of reasons, evidence, or statements of fact. A good argument (sound, or cogent) requires that accurate premises genuinely support the conclusion.

1. Deductive           
A deductive argument is constructed to produce conclusions that follow with certainty, or with logical necessity, from the premises. In a valid deductive argument, the reasoning process between the premises and conclusion is so well connected as to guarantee or ensure the conclusion. For example, if Joan is a mother, then she must be a woman. Joan is a mother. Therefore, Joan is a woman.

2. Inductive
Inductive arguments arrive at conclusions that probably follow from the premises. A strong inductive argument offers enough evidence (or reveals a pattern) to make the conclusion (highly) likely. For example, voters in Massachusetts vote predominantly for Democratic candidates over Republicans. Therefore, it’s expected that the next senator elected in Massachusetts will be a Democrat. The fact that Massachusetts citizens recently elected Republican Scott Brown to the senate shows that inductive arguments are definitely not certain!

3. Abductive
People use abductive reasoning in an attempt to arrive at the best explanation for an event or a given set of facts. Unlike deduction, abduction provides no logical certainty in its conclusions but, like induction, yields more or less probable truth.

In contrast to induction, however, abductive reasoning doesn’t try to predict specific probable outcomes. Rather, this method tries to provide the most viable explanatory hypothesis. In chapter 8 of my book Without a Doubt, I use abductive reasoning to argue that the best explanation for the life and ministry of Jesus Christ is that he was the divine Messiah.

When we take the time to organize our thoughts, it prepares us to think rationally and critically about the important issues of life. Logic is the key to this organization.

Resource: Want more tips for logical thinking? Listen to Ken’s podcast, Straight Thinking, available for free at reasons.org and iTunes.