One hour video of opening talk at the 2015 University of Arizona annual science lecture series. Throughout history, our definition of ‘life’ reflects our assumptions about how the Universe works – and why we ask the question. The ways different human cultures, ancient and current, have talked about life provide some sense of how we have defined life, and illustrate the aspects of life that fascinate us. Many cultures used life as an analog to explain the movement of winds and currents, or the motions of the planets. Today we use those mechanical systems as analogs for life. Ultimately, we may not really know what life is until we have discovered more than one independent example of it on places other than Earth – we need many diverse examples before we can generalize. But without a definition of what we’re looking for, and why we’re looking, we may have a hard time recognizing life when we find it.