When I taught psychology at university, the students with the most difficulties engaged in all or nothing thinking. The answer to many of my questions was “it depends”. Now, as a psychotherapist, I cannot count the number of clients that say they are “failing” at therapy because they cannot answer some of my questions to their satisfaction. The cool thing about therapy is you cannot fail, it is not being graded!

All or nothing thinking is taking things to the extreme. I’m either a success or failure. I’m either smart or stupid. I’m either right or wrong.

In subjects like math and science this is okay, because two plus two will always equal four. For our mental state, things are not so black and white, but various hues of gray.
Healthy eating choices is a great example. People trying to lose weight often start the day determined, but maybe at work there is a free pizza at lunch. All or nothing thinking will lead to this, “oh well, today is ruined, I’ll have to try again tomorrow.” I use this example because if you have ever tried to eat healthier, you may have experienced this thought.
I get asked, “how do I stop doing it?!” To which I say, I don’t know because we cannot always control what pops into our head. What we can control is how we respond to our thoughts. Suppose that thought pops in my head, I can choose to tell myself, “wait, I did well at breakfast and I can do well at dinner, that means most of the day I will have made healthy choices.” By being mindful of what is going through our head we can respond effectively instead of reacting in the moment, perhaps even eliminating the urge to grab a slice in the first place.
Comment with a black and white thought and what the gray area might be.