When did you reach the point where you didn't need to read another research report, didn't need to absorb another scouting analysis, didn't need to stop by the bookstore… because it simply wasn't useful or efficient to learn another thing about your field?
It's not that difficult to extend this thinking. Of course, you know enough about literature to teach literature, or enough about AC engineering to do your job. But where to draw the line? Do you know enough about the arguments and goals of political movements you're not a part of? The innovations of new restaurants in your town? The impressive new music coming out of Nigeria?
Too much too choose from, too little time.
But certainly, we don't know enough. About anything.