Knowledge Loss knowledge loss, when what was known is no longer known by anyone, is one of the costs of time and of rupture. The last person who knew how to make that tool dies. The last speaker of that language is gone. The record exists but no one can read it. The knowledge is lost. It might be rediscovered—by experiment, by finding another source, by piecing together fragments. But the direct chain is broken. First, we notice the loss. Something we used to be able to do, we can no longer do. Something we used to understand, we no longer understand. Then we can grieve it, or we can try to recover it. Recovery is not always possible. Some knowledge is gone for good. Some can be rebuilt from first principles—by observing, trying, and correcting, the way the first people who ever knew it must have learned. So the entries in this volume are not only about keeping knowledge. They are about the possibility of finding it again when it is lost. We can slow knowledge loss. We can teach. We can write things down. We can make more than one copy. We can spread the knowledge across many people. We cannot stop all loss. But we can make it less likely and we can leave clues for those who come after. When we take knowledge loss seriously, we take continuity seriously. We act as if the chain might break, and we do what we can to strengthen it. What knowledge do you have that almost no one else does? How would you pass it on?