First, I must express my appreciation for the honour of being invited by the President, Prof. Giorgio Salvini, of the Accademia Nazionale dei Lincei to give the Lezioni Lincee for 1990–1991. The added responsibility of writing a short book summarizing these lessons was also stimulating. In view of the long-standing research interests of my host at the Dipartimento di Chimica of the Politecnico di Milano, Professor Francesco Minisci, a very distinguished radical chemist, I decided to talk about my 50 years of research on the chemistry of free radicals. Some of my audience will surely demand how something that happened in chemistry fifty years ago could have any relevance to the present day. I hope to show that lessons can be learnt from the past about the philosophy of chemical research. Older chemists always seem to be interested in this subject, although many also become interested in the origins of life.

I have always tried to select a problem whose solution would be significant. On the other hand, the problem must be one that could be solved with the means at one's disposal. Frequently, I have chosen problems relevant to the chemistry of natural products.

The first problem that I discuss in this book was the major part of my PhD thesis. The problem, the synthesis of vinyl chloride, was chosen for me by the circumstance of war. It was of national importance. When the problem has been chosen, one has to think about it. The thought process is based on known literature, but the creative process is to imagine the solution to the problems in terms of the unknown.