In The Mansion of Happiness: A History of Life and Death, Jill Lepore examines life's stages, beginning before birth and ending after death, covering everything from cryogenics to breast pumps. PW Tip Sheet caught up with Lepore to find out how much we know about life and death.

You mention that our conception of life has changed from a circle to a line. What do you mean? How has our conception of life changed?

Ashes to ashes, dust to dust, is a circular idea: life ends in the same place as it begins. Broadly, in the West, the idea that life is linear began replacing the idea that life is a circle during the Enlightenment, with the idea of improvement. That era happens to have marked the beginning of the “demographic transition”: people began having fewer children and longer lives. Then came the nineteenth-century notion of progress and, later, of evolution, and, more recently, a fixation on innovation. Improvement, progress, evolution, innovation: these are linear ideas, and secular ones.

You divide your chapters along life’s stages, beginning before birth and ending after death. What stage do we understand the least? What stage is most compelling to you?

It has been said that old age is the stage of life we understand least, because it’s the only one we can never look back on. I dig that. What stage is most compelling to me? Childhood. Nothing beats a four-year-old. Except a seven-year-old.

What has been the single most important development regarding death and our understanding of it?

For the past two thousand years, very many people would say that the answer to that question is the death of Christ.

There have been estimates that the first person who will live to 150 has already been born. How does a longer lifespan change how we live?

The longer we live, the longer we die. The very old are the fastest growing population on the planet. It is a stage of life marked by the absence of children. When life was a circle, the sense that when we die our children replace us was closed the circle. When life lengthened, people began living most of their lives without children at home. This shift has outpaced anyone’s ability to reckon with its consequences, the chief of which, as far as I can see, is loneliness.

What has changed about our conception of life after death? How much is there left to learn?

No one knows what happens when we die. No one will ever know. Not knowing is the human condition. What’s left to learn is everything.