Our previous book, About Life, concerned modern biology. We used our present- day understanding of cells to 'define' the living state, providing a basis for exploring several general-interest topics: the origin of life, extraterrestrial life, intelligence, and the possibility that humans are unique. The ideas we proposed in About Life were intended as starting-points for debate - we did not claim them as 'truth' - but the information on which they were based is currently accepted as 'scientific fact'. What does that mean? What is 'scientific fact' and why is it accepted? What is science - and is biology like other sciences such as physics (except in subject