The English word logic comes from the Greek word logos. Logos can appear in arguments in a variety of ways. One way is by establishing a logical or orderly progression of ideas. One can demonstrate logos by describing the events of a story from the beginning through the middle to the end. This avoids confusing an audience. Likewise, establishing causality or citing facts and statistics in an argument demonstrates logos.


Deductive reasoning.

Using reasoning is another way to demonstrate logos. There are two main methods of reasoning: inductive and deductive. Deductive reasoning is very common in the experimental sciences. General theories give rise to hypotheses, which testing and observation either support or do not support. This method works from the general to the more specific.

Inductive Reasoning
Inductive Reasoning

Inductive reasoning is the opposite of this process. Observations are classified into patterns from which conclusions can be drawn to produce more general theories. A classic example of inductive reasoning is the literary character Sherlock Holmes, who collects observations from the world around him to create theories about the people he encounters.

Let’s look at an example. A landowner may observe that her faucet is sputtering with natural gas, her neighbors’ faucet is sputtering with natural gas and her sister’s faucet is sputtering with natural gas. She infers that all faucets in her area are sputtering with natural gas. This is not necessarily true, but it is an induction possible from the evidence she observes. Inductions are made stronger from more observations.

Or, the same landowner might engage in a type of deductive reasoning called syllogism with a very familiar structure that most people recognize: “All A is B; all B is C; therefore all A is C.” Hydraulic fracturing produces natural gas. Natural gas is in my water. Therefore, hydraulic fracturing is producing the natural gas in my water.

This deduction is logical, but it lacks the sample size and scientific robustness to be true without further investigation. Deductive syllogisms must be made from true premises. Then the conclusions must be made by logical means. Leaps in logic result in logical fallacies, which affect the truth of conclusions.

Images: “Baker Street Sign” by Sonyhamster licensed under CC BY 3.0; “Graphic” by Top Energy Training; “Faucet” by Anatolir via Shutterstock