I was writing poetry. But I found the theater such an exciting experience that one day I went home to try. I had one character say to the other guy, “Hey, man, what’s happening.” And the other guy said, “Nothing.” I sat there for twenty minutes and neither of my guys would talk. So 1 said to myself, “Well, that’s all right. After all, I’m a poet. I don’t have to be a playwright. To hell with writing plays. Let other people write plays.” I didn’t try to write a play for a number of years after that first experience.

I got quite nice results in my one-question studies, but never wrote up any of the work, because I had set myself impossible standards: in order not to pollute the literature, I wanted to report only findings that I had replicated in detail at least once, and the replications were never quite perfect. I realized only gradually that my aspirations demanded more statistical power and therefore much larger samples than I was intuitively inclined to run. This observation also came in handy some time later.

Whenever you’re making a movie, especially when you’re writing, you always have self-doubts. I did the first location shooting in Tunisia.

No, no. Well, you know, biology is such a complex system. It’s totally different from the engineering. We cannot design. Many people tried to find the therapy for cancer, but all failed. And myself, I never expected my research, working on the immune system, would lead to the cancer therapy. But, in a sense, I’m very fortunate that I also thought about it. You know, you have to try many things and if you’re lucky you can hit, but you have to pursue. That’s my feeling.

So given a Time Turner, I would tell my 21-year-old self that personal happiness lies in knowing that life is not a check-list of acquisition or achievement. Your qualifications, your CV, are not your life, though you will meet many people of my age and older who confuse the two. Life is difficult, and complicated, and beyond anyone’s total control, and the humility to know that will enable you to survive its vicissitudes.

The friends with whom I sat on graduation day have been my friends for life. They are my children’s godparents, the people to whom I’ve been able to turn in times of trouble, people who have been kind enough not to sue me when I took their names for Death Eaters.