Any competent investigator or researcher is trained to do everything possible to avoid initial thoughts or suppositions. Unfortunately, high school English teachers too often teach students to "come up with a thesis and then look for evidence to support it." That leads the general population to assume (falsely) that this is the proper way to conduct research. That kind of thinking is extremely dangerous, because a researcher starting with that mindset will overlook contradictory evidence, overemphasize supporting evidence, and twist other evidence to support the initial theory. A proper investigative frame of mind starts with a neutrally worded investigative question, not a thesis or theory. A skilled investigator looks at all research objectively until conclusions begin to emerge.
As a curriculum Director, I once reviewed a course in forensic science written by someone who actually did the kind of work shown on the highly popular CSI shows. The writer talked about the absurdity of those shows, pointing out in particular how they always form theories immediately and then set out to prove those theories true. (Particulary CSI Miami, a show which tries to make sure that they stay as far from reality as possible.)