 Formulation 1 is very similar to Turing machines but the ‘program’ is given as a list of directions which a human worker needs to follow. Instead of a one-way infinite tape, Post’s ‘machine’ consists of a two-way infinite symbol space divided into boxes. The idea is that a worker is working in this symbol space, being capable of a set of five primitive acts (\(O_{1}\) mark a box, \(O_{2}\) unmark a box, \(O_{3}\) move one box to the left, \(O_{4}\) move one box to the right, \(O_{5}\) determining whether the box he is in is marked or unmarked), following a finite set of directions \(d_{1}\),…, \(d_{n}\) where each direction \(d_{i}\) always has one of the following forms: