What is life? And what are the characteristics of living beings? Discussion about these questions has recently gained momentum. One reason is that, in the last few years, earth-like planets have been discovered. The prospects of finding extraterrestial life have thus significantly improved. But before we intensify our search for such life, we had better know what exactly we are looking for. Unfortunately, so far, the notion of life has escaped attempts to define it in a clear-cut and general way.