Look at what’s around you. Breathe the air. Take a drink of water. Listen to what’s going on. Try this while you’re inside a building sometime. Then try this while you’re outside. Do you feel any different when you do this inside versus outside?
If you’re sitting inside a building, it’s easy to feel you’re inside something because you ARE. You're getting all kinds of cues telling you you're inside. For example, perhaps the temperature is different.
Have you ever felt like you’re inside something even when there are no buildings in sight? What tells you you're outside? Though you may not feel this right away, imagine that it’s true. You’re inside something (Earth’s atmosphere), even though you’re outside of a building. Does that change your perspective in any way?
Even if you’re in the middle of a huge hay field, how is this fundamentally different from being on a movie set or the set of a play? You distinguish the two places using environmental cues. And those are ultimately your interpretation of the cues. You’re here—now—wearing a costume and playing a role. But you’re not the role. You’re the one behind the costume and the role.
If you can perceive things this way (for example, distinguishing between 'inside' and 'outside'), it means that you are able to witness them, rather than being attached to them.