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.

Arthur Conan Doyle (1859-1930) British writer and physician

“A Scandal in Bohemia” [Holmes] (1891)

Full text. Sometimes conflated with "No, no: I never guess. It is a shocking habit -- destructive to the logical faculty." from The Sign of the Four.