Skepticism (also scepticism) is, in general, a questioning attitude towards knowledge, facts and beliefs put forth as facts. It also implies doubts and reservations regarding claims taken for granted elsewhere. Philosophically speaking, the implication is that any information should be supported by evidence. Scientists, in general, can be described as empirical skeptics. They admit knowledge based on evidence but are also aware that today’s knowledge is not necessarily the ultimate truth. New evidence can always overturn what is believed to be true today.
Over time, thinkers and intellectuals for different walks of life have advocated skepticism. Perhaps Bhagwan Buddha was one of the earliest to practice and preach skepticism. At a very tender age of nine or ten he is said to have questioned the meaning of many religious customs and the usefulness of chanting hymns and mantras. One of the quotations attributed to Buddha is:

“Do not believe what your teacher tells merely out of respect to the teacher. But after examination and analysis, if it appeals to your reason and you find it to be conducive to the welfare of mankind believe it and put it to practice.”

Here are some nice quotes from other thinkers and scientists.

Rene Descartes: “If you would be real seeker of truth, it is necessary that at least once in your life you doubt, as far as possible, all things”.

Sir Bertrand Russell: “In all affairs it is a healthy thing now and then to hang a question mark on things you have long taken for granted”.

Albert Einstein: “Blind belief in authority is the greatest enemy of truth.”

Isaac Asimov: “I believe in evidence. I believe in observation, measurement and reasoning confirmed by independent observers. I believe in anything if there is evidence. The wilder and more ridiculous something is, however, the firmer and more solid the evidence has got to be.”

Robert Ingersoll: “Progress is born out of doubt and inquiry. The church never doubts, never inquires. To doubt is heresy, to inquire is to admit ignorance according to the church and it does neither.”

Albert Einstein: “The important thing is not to stop questioning. Curiosity has its own reason for existing.”

Professor Richard Feynman: "There is no harm doubt and skepticism, for it is through these that new dicoveries are made."

Swami Vivekananda: “To believe blindly is to degenerate the human soul. Do not believe in anything unquestioningly.”

The last quote perhaps sums it all. Of all the animals which inhabit the earth, man alone has the capacity to think. All great philosophers advocate that man uses this faculty fully.