Have you ever been taught that black-and-white thinking is wrong, or naive, or a sign that you're not matured mentally? Have you been taught what the statement even means? Or have you, perhaps, come to this conclusion all by yourself?
According to Wikipedia, splitting or black-and-white thinking means the inability of a person to think of someone as a cohesive whole instead of extremes: you're either all good or all bad, and everyone and every deed and every motivation is either all good or all bad, or you can have either everything or nothing. This splitting tendency is, according to psychoanalytic theory, a defence mechanism: you protect yourself by saying that people are either with you fully or against you fully; and if you're a narcissist, you and all your actions are admirable and everyone even slightly disagreeing with you is completely morally corrupted and evil.
But this is not how I, personally, see the definition of black-and-white thinking.
For me, seeing things in black and white simply means that when you do something - say something, do a deed, execute an action - it's either wrong or right in your own respective moral system. You can tell immediately if what you did was ethical. If it was, you feel good and coherent. If it wasn't, your subconscious starts to nag you about it until you've corrected your mistake. One deed is right, the opposite is wrong, and everything in between is evil - because you can make an honest mistake, and that's all right, but compromising your morals knowingly is always, well, evil.
That definition is, at the same time, not only completely black-and-white, but also inconsistent with the definition Wikipedia offers.
I wish I'd been explained younger what is meant by black-and-white thinking - that way I could have pointed out that when I see myself as a person who thinks that way, I mean my own definition.
(Does this make me a moral relativist?)