• René Descartes wrote in 1641, "I think; therefore I am." Descartes realized that we could never be sure that our perceptions are true - Your memories could be invented. Your room could be a hallucination. - But he knew with certainty that he existed. The fact that he could ask questions meant that he existed.
• About a hundred years later, David Hume showed that we could never be sure that our understanding of cause and effect is true. No matter how often something occured in the past, it is impossible to prove that it will happen in the future.
• Immanuel Kant built on Descartes' and Hume's ideas and said there is a difference between our perception of something and the thing in itself. I can see a tree, I can touch it and experience it, but I can never know the tree or experience life as the tree experiences life. I can only interpret the tree through my own senses.

All this is a way to show that whatever you believe you know to be true - you don't. Human understanding is too limited. We should then be careful what we choose to accept as true.