When you’re working with a procedurally based numerical language, there’s a lot of mysterious hidden state associated with what’s happening. For example, you have a standard program written in C, and you have various data structures, and you have subroutines that call each other and pass pointers to these data structures. If you want to look at one subroutine on its own and see what it’s doing, [to] feed this kind of input in and see what comes out, that’s pretty difficult to do in C. But in a symbolic language there’s no [problem], because whatever input might be given, you can always explicitly write it down; whatever output might come out, you can always explicitly see it. It’s always the same kind of object, always a symbolic data structure that you can explicitly see. There’s no idea that it’s some sort of mysterious pointer encoded in such and such a way.