Thoughts about the Definition of Life - RW In daily life, there is normally no real difficulty in assessing whether something is alive or not. When we look at our surroundings, we more or less automatically divide them into living and non-living objects. However, even a cursory thought will quickly bring up examples where this is less obvious. We normally base our assessment on the behaviour of the observed object and we think of a living being as something that is able to grow, develop, reproduce and die on a time scale that is compatible with our senses. Still, certain other phenomena like mountains, stars and planets have the same basic properties on entirely different scales, and what is then the difference? To me at least, "Life" is based on genes which are in turn extremely complex associations of simpler chemical substances like amino acids and nucleic acids. The crucial question, then, is whether a purely chemical definition is sufficient or not, and this is indeed where opinions are divided. Is it necessary to invoke other characteristics of (true) "life" than those which we can describe in strictly scientific terms? What is consciousness? Is life more than its material components and the interactions that can be analysed with physical means? As a natural scientist I would be inclined not to think so. On the other hand, it is also the task and moral duty of any scientist to be very sceptical. And this is why I believe that our current knowledge about physics and chemistry is certainly necessary, but by no means sufficient, to explain fully that very complex phenomenon which we call "Life". Richard M. West - ESO astronomer