Looking at a rapidly flowing stream or a thunderstorm leaves a strong visual impression, but many aspects of what is actually happening remain hidden from or are simply beyond the reach of observation, either by the naked eye or instruments. They have to be inferred from what can be observed, and this is a matter of interpretation, of imagination. It is very much the method Albert Einstein used in developing his theories of Relativity, because he could not directly observe objects moving close to the speed of light, or the movements of stars in interstellar space. In science it is called making a hypothesis, and the application of this method took modern physics far beyond empiricism (Newton had proudly claimed, “I make no hypotheses”), which was based strictly on what could be observed. Da Vinci, in this way as in others, anticipated future developments—he created hypothetical worlds that revealed the hidden structures of nature. These, in turn, helped him create paintings of great originality that are imbued with a lasting aura of conceptual power.