A far as perception is concerned, the only things with which an observer has direct and immediate contact are his or her experiences. - Alan Chalmers

The experienced and skilled observer does not have perceptual experiences identical to those of the untrained novice when the two confront the same situation.

Which facts are relevant and which are not relevant to a science will be relative to the current state of development of that science.

It should be no news to anyone that the perceptual judgements of individuals can be unreliable for a range of reasons. The challenge, in science, is to arrange the observable situation in such a way that the reliance on such judgements is minimised if not eliminated.