What is Philosophical Skepticism?

"Skepticism" derives from the Greek word for "doubt." The Skeptic is one who doubts. But, doubts what? Here we should distinguish between "ordinary skepticism," and "philosophical skepticism."

In ordinary language and circumstances, a skeptic will doubt that something exists or is true. For instance, he will, if he is a (ordinary) "religious skeptic," doubt whether God exists. Or, if he is a (ordinary) moral skeptic, he will doubt whether anything has any moral value (whatever he may mean by that.)

But a philosophical skeptic directs his doubts against knowledge. He will, unlike the religious skeptic discussed above, say, "I can know whether or not God exists.” But this is compatible with not being an ordinary religious skeptic: For the philosophical religious skeptic (unlike the ordinary religious skeptic who doubts the existence of God) may, consistent with his philosophical skepticism still believe in God! He says he knows there is a God, but that doesn't prevent him from believing in God.

The great 18th century British philosopher, David Hume is, I believe, best understood as a philosophical skeptic. He believed many things he thought it was impossible for anyone to know. For instance, he believed that there was an "external material world" beyond our senses, but also held that it was impossible to know such a thing.