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."
