Because I wanted to use it myself. I was interested in physics from a young age, and I started doing physics research when I was in my early teens, in the mid-1970s. I didn’t like doing all the mathematical calculations that were needed, and I thought it should be possible to automate them. I soon became the main user of the various experimental systems for doing mathematical computation that existed at the time, but by 1979 I had outgrown them, so I decided I had to build a system for myself.

The result was SMP, the first version of which was released in 1981. SMP ran on large computers and found users in quite a few areas, including physics. I started my first company to develop and market SMP. But quite quickly thereafter I went back to basic science, starting my explorations of cellular automata and the computational universe, and helping to found the field that’s now called complexity theory.

By 1986, though, I decided there was an opportunity to create a more powerful tool that would cover all the computation I would ever want to do. That was also a time when personal computers were beginning to be able to do serious computation. And I wanted to build a system that could bring computation to a wide audience. At the time, most physicists really didn’t use computers themselves. They would delegate computing to someone else. I was very pleased with the way that Mathematica changed that and let actual physicists compute things themselves. It was a very nice transition to watch.