"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"

Arthur Conan Doyle, through the voice of Sherlock Holmes, presents a vision of perfect rationality embodied in the "ideal reasoner". Such a person, upon receiving a single, fully illuminated fact, one revealed in every nuance and implication, could unravel not only the antecedents but also the consequences tethered to that fact. The power described is a mental mastery verging on the supernatural: to grasp a fragment of reality so thoroughly that its connections to both past causes and future effects are laid bare.

Underlying this reflection is a profound faith in the interconnectedness of reality and the potency of logical inference. Each event, each piece of evidence, exists at a nexus of causality, with innumerable threads binding it backward into history and forward into possibility. The ideal reasoner is not someone who simply observes, but one who reconstructs, piecing together the mosaic of events that converged at this point, as well as the probable ramifications that spread like ripples into the future.

The passage serves as both a celebration and a critique of pure logic. Doyle, via Holmes, suggests that the limit of deduction is not the absence of data, but the depth to which available facts can be understood. Most people observe the surface of facts, noting only their immediate significance. The "ideal reasoner" pursues each detail to its furthest logical conclusion, placing faith in the sufficiency of reason and the completeness of the observable world. They see everything as, in principle, knowable, provided the fact is inspected exhaustively.

Implied is also a comment on human limitation. While Holmes approaches such deduction, even he acknowledges that no mortal can attain complete omniscience. Still, the pursuit of this rational ideal elevates the mind, encouraging both critical thinking and humility in the face of the invisible threads connecting all facts and fates.