The definition of 'valid' is adapted by the system so that on a long term average the rate of new solutions is one per ten minutes. (The mental model of 'brute

The definition of 'valid' is adapted by the system so that on a long term average the rate of new solutions is one per ten minutes. (The mental model of 'brute force a valid hash' is often unhelpful. Most people think of "given Y find a specific X such that H(X)=Y", Bitcoin is more like "Given P,B,T find any X,Y such that H(Time||P||H(B||X)||Y)<T")