This series of articles discusses common logical fallacies (fallacy), stratagems, and language tricks. Its goal is to inform the skeptical reader on how to spot them and avoid them. This article discusses a group of common fallacies which have one thing in common – circular reasoning. Even though the individual types of circular reasoning are quite similar, they still have subtle differences.

Circular reasoning is a way of drawing conclusions from an argument which already includes the conclusion. In other words, a person makes a statement and proves it with the same statement. A typical example is the belief that certain ancient texts are true because they say they are true. For many, such a reason is enough to become a believer in a major monotheistic religion, all of which make the same claims about their unique scriptures. Another example is in the statement “Whatever is less dense than water will float, because such objects won’t sink in water” [1]. This fallacy’s Latin name is Circulus in Demonstrando.

Another type of this fallacy is a circular definition – Circulus in Definiendo in Latin. As the name suggests, such a definition uses the word it defines or its derivative, and thus becomes illogical and non-productive. One example would be defining the word “resident” as “a person who resides in a given place.” The word “residence” is the verb “reside” with a noun-forming suffix “-ence” added. In effect, the word is explained by using the same word. Scientists can also be susceptible to this kind of logical fallacy because they are often the result of careless use of language. Saying that, for example, “a species” is a group of organisms which can reproduce to have more members of the same species would be the fallacy of this kind. It is similar to saying that logic is the discipline of reasoning using the laws of logic. 

Another self-perpetuating fallacy is called Circular Cause and Consequence – Circulus in Causa et Consequens. It occurs when a cause of an effect is also described as a consequence of that same effect. For example, low spending can be blamed on a weak economy, but a weak economy can be blamed on low spending. This is a vicious circle of reasoning where neither of the factors truly explains the reason for either. There are surely other causes influencing a weak economy and, thus, consumer frugality. 

A similar circular approach is visible in investigating women for witchcraft in premodern Western societies. In a so-called “swimming test,” the accused was thrown into water. If she submerged and drowned, she was believed to be innocent. However, if she floated, she was said to be rejected by water due to being in consort with the devil. Either way, this vicious circle of reasoning ended with the woman being sentenced to death [3].

In linguistics, phrases based on circular reasoning are referred to as “tautology”. Tautology is the repetition of meaning in a phrase. It can often be found in everyday sayings, e. g. “Boys will be boys” or “let’s cooperate together.” It also appears in language cliches used in many different fields, such as the following: added bonus (journalism and marketing) [4], lived experience (media studies, social studies, psychology) [5], true fact (social media) [6], or safe haven (popular culture) [7]. To be clear, any bonus is added, all experiences are lived, facts are true by definition, and a haven is naturally safe. Tautologies are rather difficult to avoid in everyday life due to their persistence as cliches in every discipline, including scientific publications. Some other popular tautologies to be wary of include the following: each and every, end result, final outcome, advance planning, new innovation, general consensus, the exact same, natural instinct, actual experience, brief summary, and advance warning.

To sum up, knowing and recognizing circular reasoning and tautologies can not only protect the skeptic from badly justified ideas but also from bad quality writing. This is essential in the world of science and science-related journalism because circular reasoning obscures meaning and leads to the postulation of unreliable definitions and arguments. Facts are not explained just by themselves and require more than just mere repetition.