"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.
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."
to Tom Jonard's Inquiry page