Wolfram explains that the engine is built on two fundamental stepping stones: his works Mathematica and A New Kind of Science:
With Mathematica, I had a symbolic language to represent anything—as well as the algorithmic power to do any kind of computation. And with NKS, I had a paradigm for understanding how all sorts of complexity could arise from simple rules. But what about all the actual knowledge that we as humans have accumulated?
But if one’s already made knowledge computable, one doesn’t need to do that kind of natural language understanding. All one needs to be able to do is to take questions people ask in natural language, and represent them in a precise form that fits into the computations one can do…I wasn’t at all sure it was going to work. But I’m happy to say that with a mixture of many clever algorithms and heuristics, lots of linguistic discovery and linguistic curation, and what probably amount to some serious theoretical breakthroughs, we’re actually managing to make it work.