A "cognitive bias" is to the brain as an optical illusion is to the eyes. Both play tricks on the mind, but cognitive biases are far more common yet less known. A key to thinking critically is to recognize your own biases and compensate for them. For example, I happen to be very optimistic (optimism bias). Knowing this about myself allows me to apply deeper thought to my initial optimistic feelings and make a more realistic assessment of a situation.
What are your biases? How can you compensate for them?