All human activity is prompted by desire. There is a wholly fallacious theory advanced by some earnest moralists to the effect that it is possible to resist desire in the interests of duty and moral principle. I say this is fallacious, not because no man ever acts from a sense of duty, but because duty has no hold on him unless he desires to be dutiful. If you wish to know what men will do, you must know not only, or principally, their material circumstances, but rather the whole system of their desires with their relative strengths.
Bertrand Russell, The Conquest of Happiness