WHEN the National September 11 Memorial opens next week, it will be in good, if crowded, company. New York fairly bristles with memorials. They are so numerous, in fact, that no one knows exactly how many there are. Citywide estimates hover around 3,000; south of Canal Street, there are roughly 300.

Massive columns and grand arches; stolid steles and glowering busts; eagles and angels and a cavalry of horses in a state of perpetual alarm. Charging through parks, marooned on traffic islands, folded into sidewalks, the ghosts of the dead shout, whisper and plead, “Remember me.”

But we don’t, generally. Memorial mania notwithstanding, no one has ever accused New York of being overly preoccupied with its past (unlike, say, Boston or Philadelphia). Indeed, the very ubiquity of such monuments may help to ensure their invisibility — they blur into the general sensory overload. Who has time to linger before every plaque and plinth?

Before the 20th century, memorials were erected mostly to celebrate victory. Now, they are more often used to commemorate suffering — it is hard to imagine anyone building a triumphal arch these days — and have become a requisite stage in the collective grieving process, a means of dealing with cultural trauma, and perhaps a way of starting to move on. It’s almost as if, by consigning a memory to stone, we can begin to forget.