156 Are the Androids Dreaming Yet? It is the barber paradox with the word ‘set’ substituted for ‘barber’ and ‘contains’ rather than ‘shave. But it’s essentially the same logical problem. You might find this rather contrived but mathematicians must have a system totally free from paradox, otherwise there is no certainty. Frege's system was holed below the water line. Eventually, after much further work, a theory of sets was worked out that does not contain the Russell Paradox. It’s called Zermelo-Fraenkel set theory, or ZF for short. It solves the Frege problem by forbidding sets to refer to themselves. It’s a bit like Microsoft Excel’s solution to dividing something by zero. It is simply forbidden and generates an error message. Set theory was fixed and is now the basis of most mathematical thinking. What is Logic for? Logic is the foundation of mathematics. Applying it enables us to make irrefutable statements about things: numbers, lines, planes, equations and the so on — the things you learned at school - and to prove statements about these things beyond any doubt. This is not the ‘reasonable doubt’ hurdle of our law courts, but an absolute measure: No possible doubt whatever. Let’s look at one of the earliest mathematical proofs: Euclid’s proof there are an infinite number of prime numbers. Euclid created this proof in ancient Greece around 300sc — so far back that logic was in its infancy Euclid’s Elements 100AD HOUSE_OVERSIGHT_015846