What Is A Skeptic?
Often people confuse the term 'skeptic' with 'cynic' and presume the job of a skeptic is to doubt the validity of any claim, or in extreme cases, be negative about any claim. How many times have we heard someone say 'Hmm, I'm a bit skeptical about that...' when referring to such a matter?
In fact, skeptisim is one of the best tools we have to ensure we hold a world view which consists of as many true beliefs as possible, and as few untrue beliefs as possible. A skeptic arms themselves with the tools to recognise the amount of evidence which must be obtained before a particular claim can be accepted (or in some cases, rejected). Whilst that threshold may vary with every claim (depending on how much you would have to change your existing view of reality to accept said claim), frequent exposure to critical thinking topics and reasoned questioning will slowly ingrain these skills into your everyday thinking making this process much simpler.
The astronomer Carl Sagan provided a very appropriate quote on the subject: 'Extraordinary claims require extraordinary evidence.'
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With all of this in mind, skepticism (properly applied) is not biased towards any particular conclusion. It simply is a method to assist you in making judgements towards the answer which is most likely to be true whilst protecting you from accepting answers that are less likely to be true.
Unfortunately, these skills do not come very naturally to us. Our brains did not necessarily evolve for critical thinking. Instead, given the amount of information our brains have to process, it's no wonder that the brain uses mental short cuts (heuristics) and falls victim to many seemingly in-built biases (snap judgements). This allowed us to make 'quick' decisions based on our past perceived experiences, to increase our chances of survival (and arguably reduce the workload the brain has to cope with).
A good way to think about this is to imagine our more primitive selves on the African Savannah. Suddenly, we perceive a rustling in the bushes. It could be a predator, but it could be the wind. Now, is it more likely that those who instinctively fled without question were more likely to survive and pass on these traits? Whilst critically considering the likelihood of there being a predator or not is indeed more likely to lead you to the 'correct' answer eventually, it may put your survival at a disadvantage.
Thankfully the world we live in (at least for most of us) is a very different place these days!
A Definition
In 2008, Dr. Steven Novella, host of the Skeptics Guide to the Universe, wrote a very concise definition of the term 'Skeptic'.
A skeptic is one who prefers beliefs and conclusions that are reliable and valid to ones that are comforting or convenient, and therefore rigorously and openly applies the methods of science and reason to all empirical claims, especially their own.
A skeptic provisionally proportions acceptance of any claim to valid logic and a fair and thorough assessment of available evidence, and studies the pitfalls of human reason and the mechanisms of deception so as to avoid being deceived by others or themselves.
Skepticism values method over any particular conclusion.