You have a method to determine opinion and what some one does not know or can't be observed?
What does any observer have to evaluate the substance that presents itself to reason and senses? Only a single mind that resides within the observer himself. That makes for messy science.
Oh. Wait. There are other observers who cropped up by nature, all unintelligiently undesigned, who might have something to offer as corroboration. Listen to them!