There is no general theory of error, and there will never be one. Indeed, the task of theorizing error is insurmountable. The author of the entry, ‘Erreur’, in the Encyclopédie of Diderot and d'Alembert had already cautioned in the mid-eighteenth century that, although several philosophers detailed the errors of the senses, the imagination and the passions, their imperfect theories are ill-suited to cast light on practical decisions. And, he continued, the imagination and passions are enfolded in so many ways and depend so strongly on temperaments, times and circumstances, that it is impossible to uncover all the hidden forces that the imagination and passions activate. A century later the English logician de Morgan concurred, ‘There is no such thing as a classification of the ways in which men may arrive at an error; it is much to be doubted whether there ever can be’. The study of error cannot commence with a successful stroke of disambiguation upon which a coherent and all-embracing theory could be founded. Historically and etymologically ‘error’ may be traced to the Latin root ‘errare’, which originally had two contrasting meanings: first, ‘to go this way and that, to walk at random’ and second, ‘to go off the track, to go astray’. So right at the origin of the term, there is a tension between aimless wandering and straying from some definite path.