A claim you can check is one that outside evidence could settle; a claim that rests on personal values cannot be settled that way. A fact is something you can check, while an opinion is what a person thinks, and opinions can be backed up with good reasons even though a reason is not the same as a fact you can check. Whether something is a fact or an opinion does not depend on whether it is right, because a false claim that could be checked is still a claim of fact. Before you believe something, ask who says it and how they know. People knew the Earth was round long before Columbus sailed. Eratosthenes, head librarian in Alexandria, estimated the Earth's size around 200 BC by comparing the Sun's angle in two places, and he used a camel travel time of 5000 stadia from Syene to Alexandria and simple geometry to get the full circle. Columbus sailed using a guess for the size of the Earth that was too small by about a third, and a much better estimate already existed.
The idea that people use only a tenth of their brain has been linked to William James, a psychologist, who wrote that we use only a small part of our possible mental and physical resources, but no one is sure who started it. People use almost every part of their brain, and brain-imaging studies have found no part of the brain that is always switched off. At any given moment not all brain regions are firing at once, but over 24 hours most regions are continually active. A myth can last because people see their own shortcomings as proof of unused parts of the brain. The Great Wall is not visible to the naked eye from orbit, so the idea that you can see it from space is false. The wall is not visible from the Moon, and is hard or impossible to see from Earth orbit without strong lenses. The Vikings' horned helmet shows how an image from art can outlast the evidence against it: the first production of Wagner's Ring Cycle used horned helmets in its costumes in 1876, and the picture has been disproven, but it is still found in modern images.
Evaluating authority depends on context: the same author may be an expert on one subject and not another. Encyclopedias are usually tertiary sources, a collection of primary and secondary sources. Wikipedia is written by volunteers and relies on the collective wisdom of its volunteers to get the facts right, and some professors do not accept Wikipedia as a source because the volunteer approach to editing can be unreliable. The SIFT method was designed specifically for judging online sources. A conspiracy theory usually centers on a belief that events are the result of a secret plot by powerful people or groups. Conspiracy theories are not new, and they tend to grow where uncertainty, fear or distrust are common; a fraud that is proved with evidence is different from a conspiracy theory: Piltdown was a real hoax that scientists proved with tests. Where the author cites sources, check them, and look for the author's stated purpose and viewpoint, not only their facts.
In 1919 astronomers used a total solar eclipse to test whether the Sun's gravity bends starlight, as Einstein's theory said it would. Einstein's theory predicted starlight near the Sun would shift by 1.75 arcseconds, and Eddington's teams measured 1.98 and 1.61, while Newton's theory asserted a smaller bend of about 0.87 arcseconds. The test was hard: at Sobral, the telescope lens became distorted by the Sun's heat and made out-of-focus plates, and later claims that Eddington fudged the data have been shown to be incorrect. Karl Popper said the impressive thing about the 1919 test was the risk: the theory forbade an outcome that could have happened. Falsifiability means a claim could be shown wrong by some possible observation. A theory that no conceivable event could refute is not scientific, in Popper's view, and confirmations count only when they come from risky predictions. Adding untestable add-on ideas to protect a theory from refutation makes it no longer scientific. Occam's razor is a preference for simpler explanations when competing explanations fit the data equally well.
A hoax can last when it flatters what people want to be true: the Piltdown find was in Britain, which pleased some British scientists. Scientists wanted to find the first human ancestor in England, and that wish may have made them less careful about Piltdown; the hoax slowed the search for human ancestors, because other fossils did not fit it. Who made the Piltdown fake is not known for sure, as of November 2026. Confirmation bias covers how people search for, interpret and remember information so that it fits what they already believe. Percival Lowell drew maps of canals on Mars from what he saw at his observatory, and later evidence showed they were optical illusions; seeing shapes in colors at the limit of what the eye can make out can create illusions. Science can change an answer when there is new evidence: long ago Pluto was called the ninth planet, and in 2006 the IAU voted for a new definition of dwarf planet. As of November 2026 NASA lists five dwarf planets, including Pluto.
Words to know
- falsifiability: can be disproved
- tertiary: collected from others
- refute: prove wrong
- conspiracy: secret plot
- bias: leaning one way
