My father, the astronomer Carl Sagan, taught space sciences and critical thinking at Cornell. By that time, he had become well known and frequently appeared on television, where he inspired millions with his contagious curiosity about the universe. But inside the Sphinx Head Tomb, he and my mother, Ann Druyan, wrote books, essays, and screenplays together, working to popularize a philosophy of the scientific method in place of the superstition, mysticism, and blind faith that they felt was threatening to dominate the culture. They were deeply in love — and now, as an adult, I can see that their professional collaborations were another expression of their union, another kind of lovemaking. One such project was the 13-part PBS series Cosmos, which my parents co-wrote and my dad hosted in 1980 — a new incarnation of which my mother has just reintroduced on Sunday nights on Fox. 
After days at elementary school, I came home to immersive tutorials on skeptical thought and secular history lessons of the universe, one dinner table conversation at a time. My parents would patiently entertain an endless series of "why?" questions, never meeting a single one with a “because I said so” or “that’s just how it is.” Each query was met with a thoughtful, and honest, response — even the ones for which there are no answers.

One day when I was still very young, I asked my father about his parents. I knew my maternal grandparents intimately, but I wanted to know why I had never met his parents.

“Because they died,” he said wistfully.

“Will you ever see them again?” I asked.

He considered his answer carefully. Finally, he said that there was nothing he would like more in the world than to see his mother and father again, but that he had no reason — and no evidence — to support the idea of an afterlife, so he couldn’t give in to the temptation.

“Why?”

Then he told me, very tenderly, that it can be dangerous to believe things just because you want them to be true. You can get tricked if you don’t question yourself and others, especially people in a position of authority. He told me that anything that’s truly real can stand up to scrutiny.

As far as I can remember, this is the first time I began to understand the permanence of death. As I veered into a kind of mini existential crisis, my parents comforted me without deviating from their scientific worldview.

“You are alive right this second. That is an amazing thing,” they told me. When you consider the nearly infinite number of forks in the road that lead to any single person being born, they said, you must be grateful that you’re you at this very second. Think of the enormous number of potential alternate universes where, for example, your great-great-grandparents never meet and you never come to be. Moreover, you have the pleasure of living on a planet where you have evolved to breathe the air, drink the water, and love the warmth of the closest star. You’re connected to the generations through DNA — and, even farther back, to the universe, because every cell in your body was cooked in the hearts of stars. We are star stuff, my dad famously said, and he made me feel that way.