How do we know that the sky is blue?
Seems like a silly question, each of us can just look up and see that the sky is indeed blue. We can all personally verify that it is blue. Well, unless you're blind. Yet if you ask a blind man, surely he will answer you that the sky is blue and the grass is green for that is what he has always been told.
We are all blind
How do you know that there are 8 planets in the solar system? Have you ever seen them yourself? How do you know that pi is approximately 3.14? Have you ever mathematically proven that yourself? Do you know that George Washington was the 1st president of the United States? Or that your coworker properly finished that assignment? Or that the latest wikileaks release is true?
We are all blind unable to see the sky to the vast majority of knowledge. Even when it's possible to personally verify knowledge, it's still very often unreasonable. I could buy a telescope and confirm that there's 8 planets orbiting the sun, but there's no reason to. Whether it's due to a lack of time, lack of expertise, lack of resources, or the knowledge being outright unverifiable, in all facets of life, we have to trust others for knowledge.
The key question the becomes:
How do we determine who to trust?