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Arguably the most widely known astronomer of the twentieth century, and host of PBS’s Cosmos, Carl Sagan was a rare individual – a scientist who could keep up his research productivity while at the same time making science, and an understanding of it, more accessible to the public at large.
While some of his ideas would prove untennable in time, like using genetically-modified algae to terraform Venus, he had the intellectual courage to admit and learn from these failures and capitalize on them for pedagogical purposes, making brilliant use of his own mistakes to show valuable insights into the nature of science.
While often derided by others in the scientific community as a mere popularizer, Carl for many years was the Face of Science, and as one of the founding members of CSICOP, now CSI, the Committee for Skeptical Inquiry, was an accomplished figure in the skeptical movement, particularly with his books, Demon Haunted World and Broca’s Brain, and his discussions on such notions as Velikovskian astronomy, UFOlogy, Crop Circles, Alien Abduction claims and Astrology, to name a few…
While roundly criticized by proponents of these ideas for his uncompromising scrutiny of their doctrines, he also spoke on the need to keep an open mind – just not so far open that your brains fall out – and pointed out, quite correctly, that the burden of proof for extraordinary claims can work in reverse:
When one is claiming the falsehood of an idea in science that remains well-supported by the data, and is thus beyond rational doubt, then it falls upon the one to back up his assertion with evidence, for it is he who is now making an extraordinary claim, and his data must be equally extraordinary.
This is because it is conceivably possible to doubt anything, but not all doubt is rational, that is, supported by reasons, and thus while skepticism alone is untenable as a path to knowledge, credulity is equally untenable, for both of which Carl had to say —
It seems to me what’s called for is an exquisite balance between two conflicting needs: the most skeptical scrutiny of all hypotheses that are served up to us and at the same time a great openness to new ideas.
If you are only skeptical, then no new ideas make it through to you. You never learn anything new. You become a crotchety old person convinced that nonsense is ruling the world. (there is, of course, much data to support you.)
On the other hand, if you are open to the point of gullibility and have not an ounce of skeptical sense in you, then you cannot distinguish useful ideas from worthless ones. If all ideas have equal validity, then you are lost, because then, it seems to me, no ideas have any validity at all.
–Carl Edward Sagan (1934-1996), “The Burden of Skepticism” Pasadena lecture, 1987