Intellectual hobbies and careers are chosen because life without them is boring, more than because of the money — it doesn’t look like Pratchett was put off writing as a result of her father getting early onset Alzheimer’s, for example.
I.e. in traditional(*) axiomatic setup of the natural number system, "+1" is called the "successor" operation, and apart from "there being 1", we also assume that "there is a successor to 1" and then call that "2". Everything else (remarkably) flows from there: all the other numbers (whole numbers, rational, real, complex numbers), most of the Mathematical Analysis with the infinitesimals, differentiation and integrals and differential equations... After the summation is defined as an operation having a concrete set of properties, it's proven that "+1" is equivalent to "succ(1)".
(*) A more recent approach is to use set cardinality instead of the "successor" approach, but regardless of the approach, even in maths, we simply take some things for granted. I find the successor approach much more approachable for the general public though, since it's so simple, and truly requires not too much (mathematical) intelligence.