A bias is a prejudice which impedes critical thinking and makes argument less fair and reasonable. There are numerous types of bias. One may be biased against an individual, a group, or something more abstract like a philosophical or religious position. A bias is often thought of as an individual prejudice, but certain biases are widely agreed upon in particular areas, countries or periods in history.
The insidious nature of bias is that it is often unexamined by the person who holds it. As with much else in the discipline of critical thinking, the skill lies in bringing to light and examining mental processes that usually remain obscure. Imagine that you are listening to a political speech, and you quickly dismiss the arguments as nonsense. Have you really listened to the speaker, evaluated their ideas, and determined that they are not valid, or are you biased against the speaker? There are any number of grounds for bias, including the following:
1. Age: the speaker is too young to know what they are talking about, or too old to understand the world as it is today.
2. Class: the speaker sounds uneducated and ignorant or, alternatively, has an upper-class voice and appearance, suggesting a lack of empathy with ordinary people.
3. Affiliation: the speaker identifies with a political group, such as Socialist or Conservative, with which the listener do not agree, or the speaker holds a religious affiliation different from yours.
Notice that the grounds for bias given here are slightly more nuanced than many common ones, such as race, gender, sexuality or physical attractiveness.
Very few people are free from bias, so the first step in critical thinking should be to identify one's own biases and correct for them. Even someone who spends all their time evaluating arguments (a judge, for instance) will almost certainly find some people more likeable than others. To remain fair and impartial, however, a judge should be aware that they are naturally inclined towards one party on personal grounds, and refuse to allow such considerations to affect their appraisal of arguments.