 There are certain behaviours that are ‘uncomputable’—behaviours for which no formal specification can be given for a machine that will exhibit that behaviour. The classic example of this sort of limitation is Turing’s famous Halting Problem: can we give a formal specification for a machine which, when provided with the description of any other machine together with its initial state, will … determine whether or not that machine will reach its halt state? Turing proved that no such machine can be specified. (Langton 1989: 12)