Preconceived notions and discrimination

We may try and say that we do not discriminate, but we know that we actually do have some pre-conceived notions and may at times discriminate. First of all what is discriminating - nothing but unfair treatment to a fellow based on our prejudices and obviously this includes rascism, casteism, sexism etc. I don't think there can be a valid argument towards any kind of discrimination as it comes out of neither logical nor compassion. What, on the other hand does having preconceived notion mean. It is a lukewarm feeling towards a thing you are not very sure about. As I see it, it is nothing but applying probabilities. If you have been many-times lied to by a person, you will have the notion and natural instinct to not readily believe that which this person says. This is very much understandable and this behaviour comes out as a natural defence mechanism and seems to come from logic. But there is a certain blur, a muddiness in the meaning of preconceived notions. This does not include that you act on those notions - notions are just a means of putting you up on your guard. To have doubts about the certain facts/people etc.. and not to have a negative opinion. It is this blurriness which make people act more as discriminators than they really are. So the idea is simple doubt from past experiences/statistics, but verify and then believe