Thinking outside the box -- that is, avoiding preconceptions and considering unorthodox possibilities -- is important, and we all need to be reminded to do it now and then. But it's easy to overshoot. Because we always hear the phrase, we tend to imagine that it's a unilateral virtue, that it's always better to be thinking outside the box, and that's a mistake.
The reason we have to have that phrase, but no phrase to remind us to think inside the box, is not because thinking outside the box is always better. It's because we all already have a natural tendency to think inside the box. No one needs to remind us to do that. Instead, we need a constant reminder to nudge us back out of it.
But the fact is that there are a lot of good ideas inside the box, inside the realm of what is accepted and conventional, too. In fact, when you come right down to it, most of the good ideas are there. All those ideas in the box were outside the box once, but they got put inside the box because they were, at least at one time, good ideas worth keeping around. They're the ideas that already passed the test. Sure, some of them stuck around after they stopped being good ideas, after time passed them by. But that still leaves a lot more of them good than the wild mess of random untested ideas outside the box.
We have to get outside the box because we can't improve except by innovating, and there's not a lot of room to innovate from inside the box. But when we've found something outside the box that's good, it still will probably be good only in combination with things inside the box. Even if not, we have to think inside the box a lot just to know what's been done and what hasn't, and why.
But it's dangerous to get into the habit of thinking that every established idea is inferior to everything outside the box.