The differences between human beings are sometimes more striking than the similarities. There is a minority of people who, driven by a competitive urge to be the first or the best, undertake ambitious expeditions involving severe physical deprivations, high risk of fatality, and extreme isolation. Compare these people to the majority who prefer an easy, risk free life of comfort and we can see that one human being can be as different from another in their behaviour as two different species of animal. Any attempt to form general rules about human behaviour needs to bear this variation in mind.