Self-knowledge is knowing what you are really like — your character, why you do what you do, what you are good at, what you are bad at, what makes you angry or afraid. It is uncountable: 'a lot of self-knowledge', never 'a self-knowledge'. It is a calm, thoughtful, slightly serious word. You meet it in books about growing up, in psychology, in religion and philosophy, in job interviews and school reports ('the essay shows real self-knowledge'). It is almost always praised — having it is a good thing, and lacking it is the quiet criticism: 'he has no self-knowledge at all' means he cannot see himself as others see him, which is worse than being wrong about a fact. Two things students find surprising. First, it is not about knowing facts about yourself — your blood type, your birthday. It is about seeing your own patterns, honestly, including the parts you would rather not see. Second, it is not the same as confidence. A person with real self-knowledge may be quite unsure of themselves, because they know exactly where they are weak. In fact people usually say self-knowledge comes slowly, and often from being wrong, so the word carries a feeling of hard-won honesty rather than cleverness. The old Greek instruction 'know thyself' is the idea behind it.