sherlock holmes closed his eyes and placed his elbows upon the arms of his chair, with his finger-tips together. "the ideal reasoner," he remarked, "would, when he had once been shown a single fact in all its bearings, deduce from it not only all the chain of events which led up to it but also all the results which would follow from it. as cuvier could correctly describe a whole animal by the contemplation of a single bone, so the observer who has thoroughly understood one link in a series of incidents should be able to accurately state all the other ones, both before and after. we have not yet grasped the results which the reason alone can attain to. problems may be solved in the study which have baffled all those who have sought a solution by the aid of their senses. to carry the art, however, to its highest pitch, it is necessary that the reasoner should be able to utilise all the facts which have come to his knowledge; and this in itself implies, as you will readily see, a possession of all knowledge, which, even in these days of free education and encyclopædias, is a somewhat rare accomplishment. it is not so impossible, however, that a man should possess all knowledge which is likely to be useful to him in his work, and this i have endeavoured in my case to do. if i remember rightly, you on one occasion, in the early days of our friendship, defined my limits in a very precise fashion."