"And a person with clear eyes would see it and contemplate it, examining it carefully. And it would appear to them as completely vacuous, hollow, and insubstantial. For what core could there be in a lump of foam?

In the same way, a mendicant sees and contemplates any kind of form at all — past, future, or present; internal or external; coarse or fine; inferior or superior; near or far — examining it carefully. And it appears to them as completely vacuous, hollow, and insubstantial. For what core could there be in form?”

- SN 22.95 Phena Sutta