In The Middle Passage, James Hollis writes:

Grief, for example, is the occasion for acknowledging the value of that which has been experienced. Because it has been experienced, it cannot be wholly lost. It is retained in the bones and in the memory, to serve and guide the life to come. Or take doubt. Necessity has been called the mother of invention, but doubt is. Doubt may be threatening in its openness, but doubt nonetheless opens. All great advances in human understanding have come out of doubt. Even depression carries a useful message, that something vital has been “pressed down.”

Hollis, The Middle Passage, page 107

I think also of nostalgia, which indicates an unrequited longing for something. As with grief, it signals a loss, but when acknowledged it becomes less an absence than a foundation from which to build. Like a layer of sediment, studded with fossils, no longer accessible but serving as solid ground. If we try to dig it up, we threaten the stability of the future; if we let it be, it will decay and nurture us for years to come.