One concept of skepticism is that truth and knowledge are beyond peoples' reach and that therefore nothing can be known for certain!

The founder of this idea was the ancient Greek philosopher Pyrrho (365-270 BC). How he could know for certain that nothing could be known for certain is unclear. However, from Pyrrho's skepticism it followed that it's best to avoid making judgements and decisions.

Of course a decision to make no decisions is itself a decision and a very impractical one at that!

Skeptics who are less strict but more practical accept Empiricism – reliance on the senses and experience supplemented by scientific method and instruments. Results and findings obtained in this way are provisionally considered to be "truth" and "knowledge" until falsified (or unless falsified) by later scientific discoveries.

Such a skeptic would either reject or at least doubt any claim which cannot be checked through observation (or via other senses) assisted, when necessary, by scientific methods or instruments.

If an assertion is contrary to currently accepted science a skeptic would call it a "paranormal claim". These include claims that valid information can be obtained via astrology, ESP, God, numerology, superstition, tea-leaf reading, etc, and claims that there are such things, as fairies, ghosts, extraterrestrials, werewolves, etc.

Most skeptics would also reject religious claims about heaven, hell, life after death, angels, gods who answer prayer, atonement, etc. To a skeptic such claims are neither true nor false. Nor are they necessarily in conflict with science. Rather they are simply untestable and therefore to be doubted.

This, then, is the approach taken in practice by the various Associations of Skeptics although some of them call themselves "Rationalists" instead of "Skeptics".

The word "skeptic" comes from the Greek word "skeptesthai" meaning "to examine" and commonly refers to thinking for oneself and not simply accepting another's testimony.

Occasionally the skeptic has been associated with the scoffer, the disbeliever, or with the nay-sayer who rejects accepted beliefs just for the sake of being different.

However, a person can think and examine and rely on his senses and science without necessarily being a scoffer or disbeliever or even a doubter.

This can happen, for example, if what the skeptic finds out after examining the evidence agrees with what is already generally believed. Scoffing is also inappropriate if no test or observation can yet determine which side is right or wrong in a disagreement.

Another problem concerns "inductive logic" – making generalizations or predictions from past experience (including observations and trends). It's often impossible to tell which generalization is going to stay unfalsified, and yet simply being alive and conscious forces us to make inductive generalizations!

Like most people skeptics live each day and plan ahead as though tomorrow will come. In other words they have to generalize from information accessible to their senses at present, and in the past, to what will be the case in the future or in other places.

If the laws of physics stayed unchanged in the past and the sun therefore always rose predictably, most skeptics will conclude that the laws of physics will continue unchanged and the sun rise predictably! If all observed ravens were black then it makes sense to generalize and conclude that unobserved ravens are also black, until this is disconfirmed.

What would a skeptic do if a paranormal claim (a claim contrary to current science) generated reliable conclusions? What if an astrologer always made accurate predictions? Or a water diviner always finds water? Or an ancient religious book, supposedly from God, forecasts future history or future scientific discoveries? Would the skeptic generalize and predict that the astrologer, the book, or the diviner will keep on getting things right?

Such a generalization would be contrary to current science and contrary to Empiricism.

The skeptic facing such a dilemma might choose between giving up Skepticism, giving up Empiricism, giving up Science, giving up on making generalizations OR he could delete one item from his list of things to be doubted while keeping on with investigating!