Poets often express deep insights into human nature with far less verbiage than scientists. Alexander Pope’s Essay on Man, for example, is filled with pithy observations on the dualistic tensions of the human condition:



Placed on this isthmus of a middle state,
A Being darkly wise, and rudely great:
With too much knowledge for the Sceptic side,
With too much weakness for the Stoic’s pride,
He hangs between; in doubt to act, or rest,
In doubt to deem himself a God, or Beast,
In doubt his mind or body to prefer;
Born but to die, and reasoning but to err.



Pope has packed a lot into this refrain, but the final clause is an important challenge to science: Is all our reasoning for naught, to end only in error? Such fear haunts us in our quest for understanding, and it is precisely why skepticism is a virtue. We must always be on guard against errors in our reasoning. Eternal vigilance is the watchword not just of freedom but of thought. That is the very nature of skepticism.

To my considerable chagrin, it was five years into the editing and publishing of Skeptic magazine before I realized I had never bothered to define the word or even examined how others had used it. Then Stephen Jay Gould, in the foreword to my book Why People Believe Weird Things, mentioned that it comes from the Greek skeptikos, for "thoughtful". Etymologically, in fact, its Latin derivative is scepticus, for "inquiring" or "reflective". Further variations in the ancient Greek include "watchman" or "mark to aim at". Hence, skepticism is thoughtful and reflective inquiry. To be skeptical is to aim toward a goal of critical thinking. Skeptics are the watchmen of reasoning errors, the Ralph Naders of bad ideas.

This is a far cry from modern misconceptions of the word as meaning "cynical" or "nihilistic", although a consideration of the word’s history gives some insight into why its original definition has shifted. The Oxford English Dictionary offers this as its first definition of "sceptic": "one who, like Pyrrho and his followers in Greek antiquity, doubts the possibility of real knowledge of any kind; one who holds that there are no adequate grounds for certainty as to the truth of any proposition whatever. This may be true in philosophy, but not in science. There are more than adequate grounds for the probability of the truth of propositions—if we substitute "probability" for "certainty", because there are no incontrovertible facts in science if fact is a belief held with 100 percent certitude.

Superstring theory may be uncertain, but heliocentrism is not. Whether the history of life is best described by gradualism or punctuated equilibrium may still be in dispute, but the fact that life has evolved is not. The difference is one of probabilities, and this is reflected in a second usage of "sceptic": one who doubts the validity of what claims to be knowledge in some particular department of inquiry. " Okay, so we don’t doubt everything, just some things—particularly those lacking in evidence and logic. Unfortunately, it is also true that some skeptics fall into a third usage of the word: "one who is habitually inclined rather to doubt than to believe any assertion or apparent fact that comes before him; a person of sceptical temper. " Why some people are, by temperament, more skeptical than others is a subject for another essay. But suffice it to say that the reverse is also true—some folks are, by temperament, habitually inclined to believe rather than to doubt any assertion. Neither extreme is healthy.

Perhaps the closest fit to what we equate with a skeptical or scientific attitude is a fourth meaning: "a seeker after truth; an inquirer who has not yet arrived at definite convictions. " Skepticism is not "seek and ye shall find"—a classic case of what is called the confirmation bias—but "seek and keep an open mind. " What does it mean to have an open mind? It is to find the essential balance between orthodoxy and heresy, between a total commitment to the status quo and the blind pursuit of new ideas.

懷疑的價值
撰文／薛莫（Michael Shermer），《懷疑論者》雜誌（www.skeptic.com）的創辦人，著有《我們如何相信》與《邊緣科學》等書。
翻譯／高涌泉，台大物理系教授。