Chlorine is a deadly poison gas employed on European battlefields in World War I. Sodium is a corrosive metal which burns upon contact with water. Together they make a placid and unpoisonous material, table salt. Why each of these substances has the properties it does is a subject called chemistry.

All of us cherish our beliefs. They are, to a degree, self-defining. When someone comes along who challenges our belief system as insufficiently well-based - or who, like Socrates, merely asks embarrassing questions that we haven't thought of, or demonstrates that we've swept key underlying assumptions under the rug - it becomes much more than a search for knowledge. It feels like a personal assault.

There are many instances in science, where those closest to the intricacies of the subject have a more highly developed sense of its intractability than those at some remove. On the other hand, those at too great a distance may, I am well aware, mistake ignorance for perspective.

Even today the most jaded city dweller can be unexpectedly moved upon encountering a clear night sky studded with thousands of twinkling stars. When it happens to me after all these years it still takes my breath away.

In all our searching, the only thing we've found that makes the emptiness bearable is each other.

We are made of star-stuff. Our bodies are made of star-stuff. There are pieces of star within us all.

The prediction of nuclear winter is drawn not, of course, from any direct experience with the consequences of global nuclear war, but rather from an investigation of the governing physics.