- first, the correctness of the facts or conclusions I start from,
- secondly, the completeness as well as the accuracy of the data I start from,
- thirdly, the elimination of other possible or impossible conclusions from the same facts.
The fallibility of the logical reason is due partly to avoidable negligence and looseness in securing these conditions, partly to the difficulty of getting all the facts complete, most of all, to the extreme difficulty of eliminating all possible conclusions except the one which happens to be right. No fact is supposed to be more perfectly established than the universality of the Law of Gravitation as an imperative rule, yet a single new fact inconsistent with it would upset this supposed universality. And such facts exist. Nevertheless by care and keenness the fallibility may be reduced to its minimum.