What makes the indoors, indoors? It isn’t the presence of a “door” that you are “in,” unless you consider deep underground subway stations (without doors) as outdoors. But is a cave indoors? Even deep underground, fully protected from the elements, the wildness of a cave makes one hesitant to consider it truly “indoors.”
Perhaps, then, the definition of indoors and outdoors is not so definite. Some elements seem essential to the indoors: protection from the elements, especially rain, wind, sun. Beyond that, things become fuzzy. Most subway stations aren’t heated or cooled yet give the impression of indoors. We encounter many situations that exhibit some element of protection from the elements, yet aren’t fully heated or cooled, or don’t have full protection from rain, wind, and sun: amphitheaters, bus shelters, event tents, ballparks, and even greenhouses.
Rather than thinking of indoors and outdoors as a binary condition, we need to acknowledge there is a spectrum from indoors to outdoors. The concept of a spectrum is already familiar for other words we use to describe the environment around us. Although we think of “light” and “dark” as opposites, it’s easy to see that they actually represent different ends of a spectrum. When passing through a series of spaces with increasingly brighter light, at what point is the space suddenly considered “light”? The same principle applies to noisy and quiet: clear ends of a spectrum, but there is a lot of middle ground. The middle ground of indoors is so unfamiliar that English lacks vocabulary for describing it. A room might be lighter or louder, but is it more indoors or more outdoors?