From Feeling Good: The New Mood Therapy by David Burns
This refers to your tendency to evaluate your personal qualities in extreme, black-or-white categories. For example, a prominent politician told me, "Because I lost the race for governor, I'm a zero." A straight-A student who received a B on an exam concluded, "Now I'm a total failure." All-or-Nothing Thinking forms the basis for perfectionism. It causes you to fear any mistake or imperfection because you will then see yourself as a complete loser, and you will feel inadequate an worthless.
This way of evaluating things is unrealistic because life is rarely completely either one way or the other. For example, no one is absolutely brilliant or totally stupid. Similarly, no one is either completely attractive or totally ugly. Look at the floor of the room you are sitting in now. Is it perfectly clean? Is every inch piled high with dust and dirt? Or is it partially clean? Absolutes do not exist in this universe. If you try to force your experiences into absolute categories, you will be constantly depressed because your perceptions will not conform to reality. You will set yourself up for discrediting yourself endlessly because whatever you do will never measure up to your exaggerated expectations. The technical name for this type of perceptual error is "dichotomous thinking." You see everything as black and or white -- shades of gray do not exist.