memento mori

Memento mori literally means "Remember you must die". The early Puritan settlers were particularly aware of death and fearful of what it might mean, so a Puritan tombstone will often display a memento mori intended for the living. These death's-heads or skulls may strike us as ghoulish, but they helped keep the living on the straight and narrow for fear of eternal punishment. In earlier centuries, an educated European might place an actual skull on his desk to keep the idea of death always present in his mind.