How should humanity progress in the next two generations? Which challenges should we engage, in what order, and with how much sacrifice (if any) of comfort and liberty? There are probably as many distinct responses to these questions as there are thoughtful people on the planet. Not all answers are equally wise, of course, but none can be definitive either. These are ultimately questions about one's moral values and personal preferences.
Experts can help us to understand which problems are most threatening, which solutions are most promising, and how costly it might be to act or to wait. Scientists can exhort us--just as the other contributors to this special issue urge us to focus on ending extreme poverty, securing biodiversity "hot spots," improving agricultural infrastructure, boosting the efficiency of our energy use, and reining in epidemic diseases--yet the experts cannot directly steer the course of humanity.