Not all private investigators operate within the so-called "box". Consider Clayton Stoner and Larry Steele, who approached their profession using the power of the mind.
Psychology, to be specific, which each used slightly different. Stoner... no worries he wasn't really even though his fashion sense begs to differ... used various gimmicks to disorient his foes and root out the truth. Well foe... he didn't last more than one issue of Detective Comics, which itself was a reprint of a reprint of a canceled comic.
Larry Steele lasted considerably longer, though his run wasn't all that more memorable. He was around during the golden age aided by his astute psychologist pop who mentored him on some cases. However, by May 1942 Steele's short lived career, such as it had been, was done as well.