I’ve always found this idea, from the French philosopher Gaston Bachelard’s The Poetics of Space, inspiring:

“Outside and inside are both intimate—they are always ready to be reversed, to exchange their hostility. If there exists a border-line surface between such an inside and outside, this surface is painful on both sides.”

We cross the borders between inside and outside many times a day, both literally and metaphorically. There’s an intriguing tension between outside and inside; inside can imply “safe” and “included” as well as “trapped,” and outside can imply “excluded” as well as “free.” As a literal example of an artistic exploration of inside and outside, a nighttime photograph of a house with one window lit up (here, I’m thinking of the work of Todd Hido) imparts a feeling of loneliness. Is this because the viewer feels left in the dark outside of the house, or because he/she imagines the isolation of the person inside that lit room? A third possibility is that the photograph reminds the viewer of how each of us, whether inside or outside, is just one tiny light in an enormous universe.