What Evidence?

Recently, while in the car on an errand, I heard a woman say something on the radio that stopped my train of thought cold. The subject was "Faith" and the radio station was NPR.

"I do not have any answers, but I have a lot of questions. Those questions have helped define my life so much better than answers would have". (A rough paraphrase)

Hmm.

I took a moment to do a quick spot check of my own list of questions and answers. What I discovered about myself was surprising. Our unresolved "questions" do quite a lot to determine how we live. In truth, there are precious few things we will ever KNOW for certain.

What does it mean to "know" something?

The dictionary had the following entry:

1. to perceive or understand as fact or truth; to apprehend clearly and with certainty: I know the situation fully.
2. to have established or fixed in the mind or memory: to know a poem by heart; Do you know the way to the park from here?
3. to be cognizant or aware of: I know it.
4. be acquainted with (a thing, place, person, etc.), as by sight, experience, or report: to know the mayor.

This seems like another good example of how the English language fails to provide a concise definition of a simple word.
Compare #1 with #3 or #4. How can one word mean both of these things and still have any value?

Some people hear a PhD on the Discovery channel say with "certainty" that the Earth is 4.55 billion years old...and they believe him.
Why not? There is a Consensus among experts. This is something they KNOW to be true.

How?

How do you Know what you think you Know?