"Now if the laws of nature hold constant over time—if the rules and patterns of today are the same as those of yesterday and tomorrow—then a valid physical equation will depend only on elapsed time. ...Do we have a universe in which the rules of the game are fixed... or ...a universe in which they vary from moment to moment? ...Nobody knows for sure... On the largest scales, over unimaginably great intervals of space and time, the laws of nature do seem to show a sense of history. ...Newton's mechanical equations specify only differences in time, ascribing no absolute meaning to each instant. The same goes for Maxwell's electromagnetic equations. The same goes for Schrödinger's quantum mechanical equations. The same goes for Einstein's equations of relativity. The same goes for so many other equations that describe so much of the physical universe. For us, open-minded but nonetheless emphasizing this ahistorical aspect of nature, our clocks will tick and tock without any reference to any absolute starting point."