The only method we have to interpret the world around us is through previous experience or those related to us by others. The way we understand everything-- human emotion and behavior, our environment etc-- are through things we've already come to understand. If that basis of understanding is flawed then all future interpretation based on that knowledge is flawed. And since human experience and interpretation is not universal there is no way to define what a lack of prejudice would be like. The only time we are not prejudiced would only be the minute we are born. But even then our instincts prejudice us against things such as loud noises, animals etc, things that are subjectively frightening. Even locked in room you'd have experience and it would prejudice you in some form.