In response to a grief-stricken father who reported that he’d lost a eleven-year-old son to polio, Albert Einstein wrote,

A human being is part of the whole world, called by us “Universe,” a part limited in time and space. He experiences himself, his thoughts and feelings as something separate from the rest—a kind of optical delusion of his consciousness. The striving to free oneself from this delusion is the one issue of true religion. Not to nourish the delusion but to try to overcome it is the way to reach the attainable measure of peace of mind.

From Dear Professor Einstein: Albert Einstein’s Letters to and from Children (2002.)