Time is a unique dimension of the universe, along with perhaps 10 dimensions of space, according to superstring theory. Our brain and its associated consciousness perceive time as a continuous unidirectional stream. Even though we appear to be able to move freely in three dimensions of space; time's arrow for us points only one way, the present eroding into the future to leave the past behind. For two millennia, time has been likened to a stream or river. In the West, for the past two centuries, time and change have been equated almost automatically with the idea of progress. However, change always brings with it loss as well as gain, and progress is far from inevitable. This remains true in the history of medicine. The history of any subject is full of blind alleys, which, although important at the time, generally become edited out in retrospect.