What do we know?

Overheard today: "How do you know stuff daddy? How do you know stuff?"
Now, the musings of a two year old are generally not worthy of a blog post. However, this question provoked some contemplation by Yours Truly.

Most people are familiar with the concept of evidence and proof in the context of scientific theories and laboratory experiments. However, the child's question seemed to encompass all of the beliefs and knowledge that an individual collects. This caused me to wonder, for how much of what we hold to be true have we seen the evidence, or at least a reasonable justification for? How many of my actions, both big and small, are based on ideas that are either false or have no evidence to support them?

Having recently read Ben Goldacre's excellent book Bad Science, this question has been particularly fresh in my mind. Ben showed in his book how some of my beliefs on matters regarding science are false, what would the same rigorous standards of evidence and analysis reveal in other areas of my life? Furthermore, Bad Science reveals how evidence can be manipulated in order to justify that which it should not.

It strikes me that we should be more careful regarding what we allow into our minds.