Even if Watson has already witnessed Holmes's deductive skills in the past, he remains astounded by his friend's ability to glean insight about people from minor details. When Watson asks how Holmes knows that he recently got very wet and that his maid can be careless, his friend hyperbolically and ironically suggests that it is obvious:
“It is simplicity itself,” said he; “my eyes tell me that on the inside of your left shoe, just where the firelight strikes it, the leather is scored by six almost parallel cuts. Obviously they have been caused by someone who has very carelessly scraped round the edges of the sole in order to remove crusted mud from it. Hence, you see, my double deduction that you had been out in vile weather, and that you had a particularly malignant boot-slitting specimen of the London slavey."
As the friends reunite in the first part, Watson is simultaneously shocked and pleased to follow Holmes's keen senses of observation and reasoning. He is fascinated and amused as Holmes explains his process of deduction. The claim that this process is "simplicity itself" is hyperbolic. Although these powers come naturally to Holmes, he knows that for more or less all the people around him, they are everything but simple. The claim also becomes ironic when he proceeds to explain how he reached his conclusions: his reliance on obscure details for bigger-picture conclusions is neither easy nor straightforward.
On the one hand, Holmes's statement about the simplicity of his thought process gives insight into his arrogance. When he goes on to explain how he reached his conclusions, he shows not that his deduction is simple but that it is elaborate. On the other hand, Holmes's statement reminds Watson and the reader that this is merely how his brain functions. To him, it really is simple—he can't help but notice details like careless scrapes on a shoe and analyze them. Watson even admits that when hearing Holmes's explanations, “the thing always appears to me to be so ridiculously simple that I could easily do it myself." Despite this, Watson notes that "at each successive instance of your reasoning I am baffled until you explain your process."
In the second part of the story, Holmes hatches a plan to trick Irene Adler and get inside of her house. Although he remains loyal to Holmes and discreetly plays his part, Watson feels guilt over his involvement in the scheme. This reinforces the dramatic irony that marks the scene, as Watson feels bad precisely because he knows something the lovely, well-intentioned Irene Adler remains oblivious to. Watson's reflections on his guilt have the additional effect of cementing him as Holmes's foil:
I do not know whether he was seized with compunction at that moment for the part he was playing, but I know that I never felt more heartily ashamed of myself in my life than when I saw the beautiful creature against whom I was conspiring, or the grace and kindliness with which she waited upon the injured man.
Watson wonders whether it bothers Holmes to trick someone like Irene Adler. The detective's calculated manner, not to mention his proclivity for disguising himself to gather information about other people, makes it safe to assume that his conscience is not especially disturbed. Solving cases is an exciting game for Holmes, and he rarely expresses or displays compassion for others. Conversely, Watson is along for the ride but spends much more time ruminating on the consequences of his actions—moral and otherwise. The dramatic irony in this moment, and the contrasting ways their conniving affects the two men, raises the question of whether Holmes's detective talent is bolstered by his lack of empathy.
Ultimately, even if Watson feels bad, he does exactly what Holmes instructed him to do. He may feel some ambivalence over secrets and schemes, especially when it affects a seemingly innocent person like Irene Adler, but in this moment he concludes that he would rather engage in questionable moral activities than betray his friend.
And yet it would be the blackest treachery to Holmes to draw back now from the part which he had intrusted to me. I hardened my heart, and took the smoke-rocket from under my ulster.
The conflict in Watson's conscience reinforces the two men as foils for each other. Holmes appears willing to do anything for the purpose of solving a crime—even to the point of engaging in criminal activity himself. Watson, on the other hand, often reflects on how far it is appropriate to go. He is drawn to the rush of accompanying Holmes, but the plot against Irene Adler gives him pause. Their personalities and motivations complement each other: if Holmes is the duo's investigative mastermind, Watson is the the duo's conscience.