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Philosopher scientific proof long and hard the past, the differences among them the fact that hypotheses and theories. But the real point of departure is this: they are essentially the same sort of issue. All of these may be the fact that hypothesis, or the whole theory of the bombing - have advocated. Some of the ideas more credible than others. If the overwhelming evidence, we suggested that one senses the right idea, we call it a fact. But the fact is still a fact, and it is an idea.           An Idea Makes the World Go Round     Philosophers of science have argued long and hard over the differences among facts, hypotheses, and theories. But the real point is this: they are all essentially the same sort of thing. All of them—be they facts, hypotheses, or full-blown theories—are ideas. Some ideas are more credible than others. If the overwhelming evidence of our senses suggests that an idea is correct, we call it a fact. But the fact remains that a fact is an idea.   Let's take a concrete, if extreme, example that brings home this point very clearly. Consider the statement "the Earth is round." Is it a fact, a hypothesis, or a theory? A prominent creationist with whom I once spoke took offense at my suggestion that dismissing evolution as a credible notion was no different in principle from denying that the Earth is round. To him, and to most of us, that the Earth is round is a fact—something we all know to be true, something we take for granted. But why do we all think that "the Earth is round" is a fact? How many of us can perform a critical experiment to show that the Earth really is round? How many of us have ventured high enough into the upper reaches of the Earth's atmosphere that we could really see the Earth's curvature? Most of us have seen photos of the Earth taken from satellites, from spaceships, and from the moon. Clearly the Earth is round. But the relatively few vocal "flat-Earthers" have a counter even for this: to them, the spectacular achievements in space of the past half century are all an elaborate hoax—nothing more. To them, all the photos of the "Big Blue Marble" taken from satellites, space stations, and the moon itself are fakes.    Now, if the Earth is round, it is probably safe to assume it has always been so—at least since the dawn of human history, when we can pick up a written record of humanity's views on the question. Yet the roundness of Earth was certainly not generally accepted as fact when Columbus set sail with his fleet of three ships. Indeed, many people thought it was a harebrained idea, and that Columbus was about to sail over the edge of what was patently a flat Earth. Only after the globe had been safely circumnavigated a number of times without a single ship dropping off the world's side did the roundness of the Earth start to take on the dimensions of credibility we deem necessary for a notion to become a fact.   Yet Eratosthenes, a Greek living in Ptolemaic Egypt in the third century B.C., showed that the Earth could not be flat with a simple yet conclusive experiment. His predecessors had already suggested the Earth is round because it casts a curved shadow on the moon. And ships sailing toward an observer appear on the horizon from the top of the mast down, also suggesting that the Earth is curved. Hearing that the sun shines directly down a well at Syene (now Aswan, Egypt) at noon on the summer solstice (the longest day of the year), Eratosthenes measured the angle between the sun's rays and a plumb bob he lowered down a well in Alexandria, some 600 miles north of Aswan, precisely at noon. That there was an angle at all in Alexandria was inconsistent with the idea that the Earth is flat. Eratosthenes could explain the phenomenon only if he envisaged a ball-shaped Earth. Using simple trigonometry, he calculated the circumference of the Earth to be the equivalent of about 28,000 miles, a respectable approximation to the 24,857 miles our modern instruments give us today. Columbus was aware of this and of later calculations, and he used them in his navigation.   Is the proposition that the Earth is round a fact, a hypothesis, a theory, or a downright falsehood? Obviously, it is an idea that has been variously considered all four. It was first called a wild idea, then a necessarily true conclusion (albeit accepted by only a few Greek savants); its respectability as a credible idea grew with the Renaissance. Now most of us proclaim it as fact—inasmuch as all attempts to disprove it have utterly failed. Flat-Earthers notwithstanding, we now even have direct confirmatory photographic evidence that the Earth is a sphere. But a round Earth is still an idea, albeit an extraordinarily powerful idea.         — Niles Eldredge, The Triumph of Evolution: and the Failure of Creationism, Chapter 2 – Telling the Difference                     Indexes/17