 In the conservation law problem, the restrictive method described in  Section 2.1  is the only method that minimizes mind changes. Recall that the restrictive method adopts a set of conservation laws that rule out as many unobserved reactions as possible. It can be shown that if there are \(n\) known elementary particles whose reactions are observed, this method requires at most \(n\) mind changes. (The number of elementary particles in the Standard Model is around \(n = 200).\)