"You would certainly have been burned, had you lived a few centuries ago."

Early in the story, Holmes is able to guess a lot of information about Watson merely by virtue of his appearance, and Watson replies like so. Watson is clearly impressed with Holmes’s ability to glean so much information from so little, and suggests with these words that Holmes would have been considered a witch in an earlier era because of how many facts he can deduce just by observing. This suggests Holmes’s talent for logical reasoning appears almost supernatural.

“I have no data yet. It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts."

Just after Holmes reads the letter aloud, Watson asks him what it means, and Holmes’s reply reveals much about his style of Investigation. He’s well known for his ability to deduce information from a close observation of details, and this response explains why his theories are so often correct. It's important to study the facts before making a decision because making a decision first leads to incorrect assumptions and closed-mindedness. Being too focused on proving a theory leads to missing crucial details and information in an effort to bolster said theory.