People who do not have facts and reason to support their beliefs often appeal to “Common Sense” without any definition of the term or how they know it applies to the situation. A typical definition of common sense is, “sound practical judgment that is independent of specialized knowledge, training, or the like” (Dictionary.com). Most definitions include phrases equivalent to “sound judgment”, and if we accept that definition, then common sense can never be wrong. However "common sense" is often used as a substitute for sound judgment or in situations where specialized knowledge or training is required to make a sound judgment. In fact, when a person appeals to “common sense” it usually means something that the person believes to be so widely accepted that it must be true (Argument by popularity).
At one time it was common sense that the earth was flat and sat at the center of the universe, which revolved around it. This is just one example of how common sense depends on the context, knowledge, and experience of the observer.
Our world today is extremely complex and most issues we face require specialized knowledge to fully understand. For example, most people cannot make rational decisions about the efficacy of medical procedures. Not only do they not know what factors have to be considered, they do not know how to evaluate multiple, often competing, factors. “Common sense” says that if a screening procedure can identify a potentially lethal disease while it can still be treated, then it should be widely used. To reach that conclusion, however, requires statistical analysis of the accuracy of the procedure, the risk of the procedure, the actual frequency of the disease, and the risk of false positive results. Nevertheless, when experts examined all those factors and recommended reduced breast cancer screening among women without known risk factors, there was a public outcry against the facts based on “common sense” among people who could not answer, and had never even considered, the simple question, “How many breast cancers are caused by radiation from regular mammograms?”
In politics people disagree about virtually every issue, and both sides often claim “common sense” supports their side. There is often a grain of truth in commonly held beliefs, especially for simple situations. But for complex issues “common sense” is a poor substitute for knowledge and logic. It is often a smoke screen to hide a lack of knowledge or to avoid challenges to pre-existing beliefs or already decided conclusions.
As science historian Daniel Boorstin, writing about the rise of scientific methodology and its contradiction of commonly held beliefs, noted, “Modern Western science takes its beginning from the denial of this commonsense axiom… Common sense, the foundation of everyday life, could no longer serve for the governance of the world.” (The Discoverers, A history of man’s search to know his world and himself)