If someone said to you, "List five scientific facts," you could probably do so with little difficulty. Living things are composed of cells. Gravity causes things to fall. The speed of light is about 186,000 miles/second. Continents move across the surface of
Earth. Earth revolves around the sun—and so on. Scientific facts, most people think, are claims that are rock solid, about which scientists will never change their minds. Most people think that facts are just about the most important part of science, and that the job of the scientist is to collect more and more facts.
Actually, facts are useful and important, but they are far from being the most important elements of a scientific explanation. In science, facts are confirmed observations. When the same result is obtained after numerous observations, scientists will accept something as a fact and no longer continue to test it. If you hold up a pencil between your thumb and forefinger, and then stop supporting it, it will fall to the floor. All of us have experienced unsupported objects falling; we've leaped to catch the table lamp as a toddler accidentally pulls the lamp cord. We consider it a fact that unsupported objects fall. It is always possible, however, that some circumstance may arise when a fact is shown not to be correct. If you were holding that pencil while orbiting Earth on the space shuttle and then let it go, it would not fall (it would float). It also would not fall if you were on an elevator with a broken cable that was hurtling at 9.8 meters/second2 toward the bottom of a skyscraper—but let's not dwell on that scenario. So technically, unsupported objects don't always fall, but the rule holds well enough for ordinary use. one is not frequently on either the space shuttle or a runaway elevator, or in other circumstances in which the confirmed observation of unsupported items falling will not hold. It would in fact be perverse for one to reject the conclusion that unsupported objects fall just because of the existence of helium balloons.
Other scientific facts (i.e., confirmed observations) have been shown not to be true. Before better cell-staining techniques revealed that humans have twenty-three pairs of chromosomes, it was thought that we had twenty-four pairs. A fact has changed, in this case with more accurate means of measurement. At one point, we had confirmed observations of twenty-four chromosome pairs, but now there are more confirmations of twenty-three pairs, so we accept the latter—although at different times, both were considered facts. Another example of something considered a fact—an observation— was that the continents of Earth were stationary, which anyone can see! With better measurement techniques, including using observations from satellites, it is clear that continents do move, albeit very slowly (only a few inches each year).
So facts are important but not immutable; they can change. An observation, though, doesn't tell you very much about how something works. It's a first step toward knowledge, but by itself it doesn't get you very far, which is why scientists put it at the bottom of the hierarchy of explanation.
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