Thinking outside the box

Suddenly, I recalled what my friend said to me long long time ago:" People say we have to think out of the box, but why do we have to think with a box? We have to think without a box..."

Till now, I cannot really understand how to practice what he shared: to think without a box.

Is the 'box' referring to 'perception'?

We think we are thinking outside the box only because we cannot see how big the box really is. Imagination cannot easily transcend the boundaries of the present, and one reason for this is that it must borrow machinery that is owned by perception. Daniel Gilbert.