"The human imagination, including the creative scientific imagination, can ultimately function only by evoking potential or imagined sense impressions... I confess I never met an experimental physicist who does not think of the hydrogen atom by evoking a visual image of what he would see if the particular atomic model with which he is working existed literally on a scale accessible to the sense impressions - even while realizing that in fact the so-called internal structure of the hydrogen atom is in principle inaccessible to direct sensory perception. This situation has far-reaching consequences for the method of experimental investigation."