In the analytic method, knowledge is obtained by use of deductive and non-deductive rules. Since such rules are not plausibility preserving, they may give rise to errors. On this basis, contrary to a tradition according to which error is heterogeneous to knowledge, this chapter maintains that error is homogeneous to knowledge and inherent to it. Indeed, error is even homogeneous and inherent to logical and mathematical knowledge. The chapter also maintains that, though error may be a serious problem for logic, mathematics and science, it may also be fruitful. For, an analysis of the causes of error may provide useful indications for forming new hypotheses, and may also help to formulate new concepts.