(2) The author was badly situated for observing. The practice of the sciences teaches us what are the conditions for correct observation. The observer ought to be placed where he can see correctly, and should have no practical interest, no desire to obtain a particular result, no preconceived idea about the result. He ought to record the observation immediately, in a precise system of notation; he ought to give a precise indication of his method. These conditions, which are insisted on in the sciences of observation, are never completely fulfilled by the authors of documents. It would be useless, therefore, to ask whether there have been chances of inaccuracy; _there always have been_, and it is just this that distinguishes a _document_ from an _observation_. It only remains to look for the obvious causes of error in the conditions of observation: to inquire whether the observer was in a place where he could not see or hear well, as would be the case, for example, with a subordinate who should presume to narrate the secret deliberations of a council of dignitaries; whether his attention was greatly distracted by the necessity for action, as it would be on the field of battle, for example; whether he was inattentive because the facts had little interest for him; whether he lacked the special experience or general intelligence necessary for understanding the facts; whether he analysed his impressions badly, or confused different events. Above all, we must ask when he _wrote down_ what he saw or heard. This is the most important point: the only exact observation is the one which is recorded immediately it is made; such is the constant procedure in the established sciences; an impression committed to writing later on is only a recollection, liable to be confused in the memory with other recollections. _Memoirs_ written several years after the facts, often at the very end of the author's career, have introduced innumerable errors into history. It must be made a rule to treat _memoirs_ with special distrust as second-hand documents, in spite of their appearance of being contemporary testimony.