Throughout the history of philosophy there has been a sustained interest in the concepts of knowledge, truth and meaning; interest in the concepts of error, falsity and nonsense, on the other hand, has been intermittent and spasmodic. Error, for example, has suffered at the expense of knowledge to such an extent that sometimes its very existence has been denied, or it has been explained away as being merely the absence of or privation of knowledge; many theories of truth are so constructed that no place can be found for falsity, and theories about what constitutes making sense pay, on the whole, little heed to what constitutes nonsense. In this paper I hope to do something to redress the balance so far as error is concerned. My remarks are prompted by the hope that, just as we may best understand health through the study of disease, so a consideration of error or failure may throw light on knowledge or success. It is clearly not very informative to say of error, falsity and nonsense that they are merely the absence of knowledge, truth and sense; indeed it is just as laconic as a proposed medical definition of disease as the absence of health.