Basically, a great deal of human thought processing, judgments, etc. is affected by irrelevant surroundings. For one example, you give two different groups of people the same essay to read. If the presenter of the essay gives an endorsement to one group before the student reads the essay, and the disparagement to the other group, the group that heard the endorsement will almost always give more favorable opinions of the essay; ye3t both groups will say they were not affected by the presenter. For all we like to think we rationally determine the worth of what we read, who says something, how other people thank about that speaker, etc. have massive influences on how we understand information. That is profoundly non-rational.