Arthur Conan Doyle frequently started his Sherlock Holmes stories with demonstrations of Sherlock Holmes' powers of deduction. In the case involving Jabez Wilson, Holmes explains his deductions about Wilson to the client himself. He knew that Wilson had been a manual laborer because he observed that one of Wilson's hands was larger than the other. Wilson tells him that he began as a ship's carpenter. Since Wilson did a lot of traveling on ships, that would explain how he had gotten to China. Holmes tells him:
“The fish that you have tattooed immediately above your right wrist could only have been done in China."
Wilson, Holmes explains, is also wearing a small Chinese coin on his watch-chain. Holmes has also observed that Wilson is wearing the Freemason arc-and-compass pin. He declines to explain how he knows Wilson takes snuff, but it should be obvious to the reader that a snuff-taker would drop some grains of snuff on his clothing, especially on his vest. Watson has observed that Wilson is wearing "a not over-clean black frock-coat." He does not take good care of his clothes. There would be plenty of evidence of snuff-taking on the vest, frock-coat, and trousers.
Wilson has certainly been doing a lot of writing lately. He has been copying the Encyclopedia Britannica at the offices of the Red-Headed League for the past eight weeks. This is what he has come to see Holmes about. Holmes explains how he knows that Wilson has been doing a lot of writing:
“What else can be indicated by that right cuff so very shiny for five inches, and the left one with the smooth patch near the elbow where you rest it upon the desk?”
The author wanted to account for an obvious question that would occur to his readers. If Wilson's new assistant, who calls himself Vincent Spaulding, is always going down to the cellar supposedly to develop photographs, why has Wilson never had sufficient curiosity to go down to take a look. After all, it is his own cellar. Watson describes Wilson as follows when he first meets him:
Our visitor bore every mark of being an average commonplace British tradesman, obese, pompous, and slow.
Wilson is also elderly. Such a man would not be likely to climb down his steep cellar steps and back up again out of mere curiosity. The fact that he takes snuff seems intended to suggest that he would also be short of breath. Snuff is a finely ground tobacco inhaled through the nose. Like all tobacco, snuff should have a deleterious effect on the lungs. We see an example of Wilson's addiction during the interview:
“Your experience has been a most entertaining one,” remarked Holmes as his client paused and refreshed his memory with a huge pinch of snuff. “Pray continue your very interesting statement.”
Note that it is "a huge pinch." He must be heavily addicted. There would surely be traces of snuff all over his clothes. Such a man would be highly unlikely to want to climb up and down a flight of cellar steps. It was fortunate for him that he never did so, because he would have caught John Clay digging a tunnel, and Clay would have had no qualms about murdering his employer with the shovel he was using for his digging and burying the old man's body in his own cellar.
Watson is no longer sharing rooms at 221B Baker Street with his friend Sherlock Holmes. In the opening sentence Watson writes:
I HAD CALLED upon my friend, Mr. Sherlock Holmes, one day in the autumn of last year and found him in deep conversation with a very stout, florid-faced elderly gentleman with fiery red hair.
Note that Watson describes Wilson as "florid-faced" as well as stout and elderly. Wilson's red face strongly suggests high blood pressure. This would be another reason why Wilson would not want to climb up and down his cellar steps. If he did so he would end up gasping for breath and experiencing heart palpitations. Conan Doyle takes pains to explain why Wilson has never found out that his assistant was digging a tunnel right under his nose, so to speak. There is a fourteen-year-old girl living on the premises who does "a bit of simple cooking and keeps the place clean." Clay could have warned her never to go down into the cellar or he would see that she lost her job. Clay himself apparently lives on the premises too. He may be able to do some additional digging while Wilson is asleep. But Wilson is an obvious hindrance to Clay, and time is of the essence. That is why he and his partner, who calls himself Duncan Ross, invented the Red-Headed League to get Wilson out of the way while both of them worked on the tunnel.