...doubt is not to be feared but welcomed and discussed....
--Richard P. Feynman
From Abracadabra to Zombies
In a nutshell: Scientific skepticism holds that science is the best way to find out things about the world and ourselves. Scientific skeptics don't trust claims made by people who reject science or who don't think that science is the best way to learn about the world.
Scientific skepticism thinks that the best way to find out what makes things tick in the universe is the way of science. Science uses reason, logic, observation, experiments, and math to discover how things work.
When it comes to finding out what makes the universe tick, other ways have been tried but none of them have been as successful as the ways of science. People have tried using their dreams or the guts of birds to find out what's going on around them. Some have tried eating stuff that gives them visions. Some just made up stories that seem to explain things (like the story about a mountain being what was left over after two giants had a battle). Some say they got knowledge about the universe from non-human creatures. The methods of science aren't perfect, but they're better than any of the other ways of knowing the world that people have tried.
We know that science can't be perfect and give us absolute certainty. There are limits to what we can know by seeing, hearing, touching, tasting, and smelling (perception). To know things we have to remember other things, but memory is not perfect. Scientists must trust other scientists to be honest and accurate. But our brains can trick us. And some people trick others and don't always tell the truth. Even so, the results and discoveries of science have proved that its methods are the best ways we have for studying nature. Best of all, science is self-correcting: when a scientist makes a mistake, other scientists will find it in time.
Scientific skeptics don't trust people when they say that some other way of knowing is better than science. Scientific skeptics don't trust extraordinary claims, claims that go against everything science has found to be true. One extraordinary claim says that some medicines work wonders even though they are just water (homeopathy). Another claims that planets have personalities (astrology). Another claims that bracelets made of plastic can give you power (magical thinking about jewelry and ornaments). Another claims that the lines on a person's hand (palm reading) or rear end (rumpology) can tell us what her future will be like.
Scientific skeptics sometimes say that extraordinary claims require extraordinary evidence. Extraordinary claims can be true only if almost every scientist in the world is wrong. Scientists have been wrong, of course, and there have been times when an extraordinary claim has become an ordinary claim backed by lots of evidence. Remember that at one time most scientists thought that the Earth was only a few thousand years old and that diseases could be cured by bloodletting. We now know that draining blood to let out "bad humors" is not going to cure any diseases. We also now know that the Earth is about 4 billion years old. (To get some idea of how old that is, think of this: if you were born four billion seconds ago, you would be more than 125 years old.)
Scientific skeptics don't say that all extraordinary claims are false. A claim isn't false just because it hasn't been proven true. Maybe someday someone will prove an extraordinary claim is true, but until the evidence is in scientific skeptics think we should not give up on the tried and true ways of science on the off chance that some extraordinary claim might be true. It's possible pigs can fly, but until we see the evidence we shouldn't give up what science has taught us about pigs and flying.
It is true that science doesn't know everything and can't answer all our questions. Using reason, logic, math, and observation won't solve every problem we run into. But an extraordinary claim doesn't come any closer to being true just because science doesn't know everything.
skeptics as investigators
The word 'skeptic' comes from a Greek word meaning inquirer. An inquirer is someone who investigates things and asks questions. Sometimes the questions that skeptics ask might seem rude. A skeptic will not just take a person's word for it if she claims that she is psychic and "sees" that a person's dog is in heaven romping around with other ghosts of dogs. The skeptic will ask "how do you know that?" If the psychic says, "my spirit guide Leroy told me," the skeptic asks "why should we believe there are spirit guides and why should we believe you are telling the truth?" If you become convinced that there are spirit guides, you should still ask "how do we know Leroy is telling the truth?"
Even if the psychic is very famous and has been on many television shows, the skeptic will not just take her word on faith. We can't know for sure whether a person gets revelations from spirits, but if she claims she does, the scientific skeptic will ask for some proof. If the psychic says her predictions have been right many times, the skeptic will investigate her claim. In fact, many skeptics have investigated many famous psychics and have found that they don't always tell the truth about their predictions being accurate.
Scientific skeptics know that millions of people believe extraordinary claims even though they have not investigated these claims themselves. Skeptics often investigate why people believe extraordinary claims about psychics, aliens, water medicines, magic jewelry, and other weird things.
Scientific skeptics even investigate people who call themselves skeptics but who really are either denialists or contrarians.
A denialist is someone who denies something is true that most scientists think is true. A denialist often gives a long list of twisted facts and brings up things that might be true, while leaving out many facts. For example, denialists have twisted the facts and left out many things to show that cigarette smoking is safe, that evolution is a hoax, that vaccinations are not safe, that 9/11 was a plot by the Bush administration, and that the Apollo moon landing was a hoax.
A contrarian is someone who won't accept that something is true because it's not absolutely certain. Scientific skeptics don't reject claims just because they are not absolutely certain. Some claims have been proven beyond a reasonable doubt. Scientific skeptics know that most scientists could be wrong about such things as global warming, cigarette smoking, and vitamins. But when the majority of scientists agree, for example, that the evidence shows that cigarette smoking causes lung cancer, the scientific skeptic doesn't reject the claim simply because there is some possibility that some study in the future will show that they're wrong.
The history of science shows that scientists have rejected many claims that they once thought were true. When the evidence and the arguments show that a long-held belief in science is wrong, scientists change their minds. What do you do when you find out you were wrong about something?
Finally, scientific skepticism does not trust every claim made by scientists. We are right to not trust the claims of those who don't use scientific methods correctly or who cheat. The history of science is full of corrections because it is full of errors. It may seem strange, but our success in getting more and more knowledge about the universe is because scientists question both nature and the work of other scientists.
To be a scientist, you have to love learning new things about the world. You have to love trying to figure out what makes things tick.
To be a scientific skeptic you have to love science and be ready to question what people say no matter who they are.