It’s about studying the computational universe of all possible programs and understanding what they can do. Exact science had been very focused on using what are essentially specific kinds of programs based on mathematical ideas like calculus. My goal was to dramatically generalize the kinds of programs that can be used as models in science, or as foundations for technology and so on.

The big surprise, I suppose, is that when one just goes out into the computational universe without any constraints, one finds that even incredibly simple programs can do extremely rich and complex things. And a lot of the book is about understanding the implications of this for science.

I’ve been very happy with the number and diversity of people who I know have read the book. There’ve been thousands of academic papers written on the basis of it, and there’s an increasing amount of technology that’s based on it. It’s quite amazing to see how the idea of using programs as models in science has caught on. Mathematical models dominated for three centuries, and in a very short time, program-based models seem to have become the overwhelming favorites for new models.

When the book came out, there was some fascinating sociology around it. People in fields where change was “in the air” seemed generally very positive, but a number of people in fields that were then more static seemed to view it as a threatening paradigm shift. Fifteen years later that shift is well on its way, and the objections originally raised are beginning to seem bizarre. It’s a pity social media weren’t better developed in 2002, or things might have moved a little faster.