The notion of a _fact_, when we come to examine it precisely, reduces to an affirmative judgment having reference to external reality. The operations by which we arrive at such a judgment are more or less difficult, and the risk of error is greater or smaller according to the nature of the realities investigated and the degree of precision with which we wish to formulate them. Chemistry and biology need to discern facts of a delicate order, rapid movements, transient states, and to measure them in exact figures. History can work with facts of a much coarser kind, spread over a large extent of space or time, such as the existence of a custom, of a man, of a group, even of a people; and these facts may be roughly expressed in vague words conveying no idea of accurate measurement. With such easily observed facts as these to deal with, history can afford to be much less exacting with regard to the conditions of observation. The imperfection of the means of information is compensated by a natural faculty of being satisfied with information which can easily be obtained. Documents supply little else besides ill-verified facts, subject to many risks of falsehood or error. But there are some facts in respect of which it is very difficult to lie or be mistaken. The last series of questions which the critic should ask is intended to distinguish, in the mass of alleged facts, those which by their nature are little subject to the risk of alteration, and which are therefore very probably correct. We know what, in general, are the classes of facts which enjoy this privilege; we are thus enabled to draw up a list of questions for general use, and in applying them to any particular case we ask whether the fact in question comes under any of the heads specified in advance. (1) The fact is of a nature to render falsehood improbable. A man lies in order to produce an impression, and has no motive to lie in a case where he believes that the false impression would be of no use, or that the falsehood would be ineffectual. In order to determine whether the author was in such a situation there are several questions to be asked.