I collect in this appendix some simple reflections on scientific methodology, and on the content of scientific theories, relevant for quantum gravity. In particular, I try to make more explicit the methodological assumptions at the root of some of the research described in this book, and to give it some justification.

I am no professional philosopher, and what follows has no ambition in that sense. I am convinced, however, of the utility of a dialog between physics and philosophy. This dialog has played a major role during the other periods in which science has faced fundamental problems. I think that most physicists underestimate the effect of their own epistemological prejudices on their research. And many philosophers underestimate the effect – positive or negative – they have on fundamental research. On the one hand, a more acute philosophical awareness would greatly help the physicists engaged in fundamental research. As I have argued in Chapter 1, during the second half of the twentieth century fundamentals were clear in theoretical physics and the problems were technical, but today foundational problems are back on the table, as they were at the time of Newton, Faraday, Heisenberg and Einstein. These physicists couldn't certainly have done what they have done if they weren't nurtured by (good or bad) philosophy. On the other hand, I wish contemporary philosophers concerned with science would be more interested in the ardent lava of the fundamental problems science is facing today. It is here, I believe, that stimulating and vital issues lie.