"Know yourself" is, I'd say, a rather sensible piece of advice - after all, it's not a bad idea to know approximately what one likes or dislikes, and how one normally reacts, and so on.
However, it looks as if trying to explore one's self in order to get to know it better does not necessarily improve self-knowledge. Or so this study found out - and if you're intrigued now, you can go read it, as it's open access.