The word "bias" is from the French word biais, "slant", or "slope". An incline is slanted. A person who is biased leans in a particular direction. Take politics as an example, a journalist who "leans" left is biased to the left, a journalist who "leans" right is biased to the right.
So what exactly does it mean to say that everything we say is biased? Let's assume for a moment that this is true; everything, regardless of the truth claim, is said within the framework of a bias, an inclination, a slant or angle. What exactly is being claimed here? Does it mean that we simply cannot trust what anyone says, because it lacks objectivity? Does the claim "everything we say is biased" include the claim itself?
If it is true that absolutely everything we say is biased and there's no escaping it, then the statement is a truism. Sure, everything we say is biased; now let's get on with the real task of determining whether what I am about to argue regarding this historical point or that moral claim is true or not.
If it means that we simply cannot trust what anyone says, because it lacks objectivity, then we have to ask how it is possible to achieve objectivity. How can I escape from my own subjectivity in order to acquire an unbiased grasp of what it is I am trying to understand? The problem, however, is that I cannot escape from my subjectivity. An object of knowledge is only such in relation to a knowing subject. Subject and object are correlative terms. Moreover, there is a two-fold mode of knowing in the human person: sensation (which we share in common with brute animals), and intellection (which we share in common with angels). "Nothing is in the intellect that is not first in the senses", said Aristotle. All knowledge begins in sensation, but ends in intellection, that is, in the apprehension of simple universal concepts, which when combined become judgments that are either true or false (i.e., all men are rational), and which in turn become premises to syllogisms or arguments which end in a conclusion (i.e., therefore, it follows that everything anyone says is biased).
What this means ultimately is that human knowledge participates in the limitations of sense perception. We gradually come to understand the natures of things through their activity (we observe how a chemical reacts, or how an animal behaves, etc). Our knowledge of the nature of a thing is not instantaneous, but gradual. Moreover, we only see the world through a particular vantage point. If you look out the window of the classroom every day, you become familiar with the neighborhood from a particular angle; you don't see it from the south of where you are, looking north; someone else does, namely the one who lives in that house over yonder. As a result, you see things that she might not, while she sees things that you might not.
Similarly, someone growing up under an oppressive regime will experience freedom here in Canada much differently than the spoiled child who was raised in freedom and privilege. Both are biased, that is, both see the world from an angle (a slant). And so, it is simply not possible to see the world from an unbiased perspective. To do so, one would have to be an angel, an immaterial substance of a rational nature - or God.
Does that mean, however, that it is no longer possible to say anything that is objectively true? Of course not. In fact, it is often the case that a bias is exactly what one needs in order to be able to finally see the truth. How many people thought a certain way when they were young, but changed years later after more life experience? How many of us were highly critical of certain people, like our own parents, when we were young adolescents, but upon experiencing the difficulties of parenting our own children, came to the realization that "I had no idea what I was talking about back then"? What happened? We began to see parenting from a different angle, from a different vantage point, that is, with a different slant, namely, from the angle of a parent with a host of responsibilities, fears and uncertainties, that were simply absent in childhood. That "angle" or slant (bias) opened up a whole new world that initially we had no idea existed. Only within that framework was it possible to finally understand our parents' behaviour.
St. Thomas Aquinas speaks of two kinds of moral knowledge: scientific and connatural. A morally good man who never read a book or took a course on ethics can, however, possess moral knowledge by a kind of connaturality (or knowledge through inclination). This means that he or she knows the right course of action in a given circumstance through an interior "leaning". She knows from within, for she is inclined to choose the morally noble course of action because she possesses the virtues. This is what Aquinas is referring to in the following: "Now rectitude of judgment is twofold: first, on account of perfect use of reason, secondly, on account of a kind of connaturality with the matter about which one has to judge. Thus, about matters of chastity, a man who has learnt the science of morals judges rightly through inquiry by reason, while he who has the habit of chastity judges rightly of such matters by a kind of connaturality."
Consider an Olympic figure skating or gymnastics competition. To the rest of us who are not skaters, the competitors all look equally skilled, for the most part at least. But to the judge, who was a former skater or gymnast, there are obvious grades of perfection within the ranks of the competitors. The judge is able to correctly judge of the quality of each skater by a kind of connaturality; he or she knows "from within", for he or she has become athletic, has acquired the skill or quality of being a skater or gymnast, and possessing that quality from within enables him or her to judge rightly concerning matters pertaining to that specific sport. So too, a person who has become musical will be able to judge the performance of a piece much better than one who is not at all musical, and a person who is a skilled artist will be able to judge a work of art much better than one who is not at all artistic. Even a critic who has no artistic talent will be able to judge a work of art properly as long as he has a great love of art and has experienced a great deal of it, for love brings about a mutual indwelling. Some people say they "live and breathe" art, or music, or sports, etc. In other words, it is within them. Their knowledge of what constitutes great art, or music, or literature, etc., stems from their very depths.
And so it turns out that bias is really the only way to objective truth. An unbiased observer would make a lousy judge of music, or art, or gymnastics, and a person lacking the virtues (which are inclinations or biases), such as a criminal, would make a lousy judge of moral character.
It is not bias that blinds the intellect, but the wrong bias that impedes right judgment. We wouldn't have the mother of one of the competitors judge the competition, because often we see what we want to see - a mother might not see the flaws in her daughter's performance. She has the wrong bias for judging a gymnastics competition - love for her daughter does not equip her to judge a sport, but it does enable her to judge a host of other things regarding the child (i.e., how best to speak to him/her in this or that situation, what the most fitting Christmas present would be, etc).
So too, who best to teach a subject like Church history? Someone whose ruling passion is anti-Catholicism? Or someone who loves the Church? The answer is not all that simple. The one who has no love for Catholics might be more willing to look at the sins of the Church throughout the centuries; and so his presentation of Church history might be more complete than, say, the one who simply cannot look objectively at the difficult facts. But it is also true that the one who despises Catholicism might not be willing to look more deeply into the causes and conditions of those difficult facts for fear of discovering what he would rather not discover, and that the one who loves the Church is more willing to dig more deeply into the mire to better grasp the facts of history. Both have a bias, and that is inevitable, but it is not always clear what bias is acting as an impediment and what is acting as a channel to a more complete knowledge.
An honest scholar is one who loves truth more than he loves himself, and so is one who has become aware of his biases. He is one who will allow the right biases to take him more deeply into the complexities of the discussion in order to find the treasure he's looking for, and he will not allow the wrong biases, his own bigotries perhaps, to keep him from listening to those who can open him up to a world that he otherwise would not have known.
What all this suggests, it seems to me, is that Aristotle was right when he said that it is not possible to acquire the intellectual virtues without first possessing the moral virtues, such as honesty, which is a part of justice. He must have been advanced in years to have had such an insight.