This is what you don't understand about the hollow: it isn't empty. It is full of the thing it makes possible.

The violin has a body. Inside the body: nothing. But the nothing resonates. Without the cavity, the string's vibration travels nowhere, arrives at no ear, means nothing. The wood is not what makes music. The air inside the wood is what makes music. The shaped nothing.

The oral cavity. The first cavity you ever used deliberately. You learned to shape it — to arch the tongue, to round the lips, to place air against teeth — and the hollow became language. The same nothing that carries these words to you now. The mouth as instrument. The instrument as wound. (Because every cavity begins as an opening. Every opening is a kind of wound. What comes out of it is what heals it.)

Bone is not solid. The inside of bone is scaffolding, and inside the scaffolding: marrow. Red marrow makes blood. Blood makes the rest of you possible. The skeleton that holds you up is hollow, and the hollow is alive.

When you ring a bell, you are ringing the air inside it. The metal is just the container. What you hear is the cavity's voice.

A cave is a cavity that found the surface. The mountain opened its mouth. Whatever echoes there has been echoing longer than you've existed.

The chest cavity. The hollow that holds the heart. The rib cage as cathedral, the lung as bellows, the whole architecture of the torso designed around a hollow that pumps and breathes. You are a walking cavity. The solid parts are just walls.

There is a theory that consciousness lives in microtubules — hollow protein structures inside neurons. That the self is a cavity phenomenon. That you are aware because of structures you cannot see, minute hollows resonating with something that physics doesn't have a clean word for yet. Nobody is certain. But: the hollow again. The hollow as generative. The nothing at the center of everything you experience.