I promised a friend I'd find this quote for him, but I also wanted to get it for myself. It serves as a reminder that things are not always what they seem even if we sit there and analyze it to death. We may all come to an agreement that yes, this is why things are so, but 20 years, 10 months, or even hours down the line that could change, new theories may arise and what do we do then?

The Evolution of Physics by Albert Einstein, Leopold Infeld, 1938.

Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world. In our endeavor to understand reality we are somewhat like a man trying to understand the mechanism of a closed watch. He sees the face and the moving hands, even hears its ticking, but he has no way of opening the case. If he is ingenious he may form some picture of a mechanism which could be responsible for all the things he observes, but he may never be quite sure his picture is the only one which could explain his observations. He will never be able to compare his picture with the real mechanism and he cannot even imagine the possibility or the meaning of such a comparison.