Weak psychological egoism is the doctrine that anything an agent does intentionally, that agent does at least expecting thereby to realize one of her self-regarding ends. (Strong psychological egoism, by contrast, is the doctrine that agents act always intending thereby to realize a self-regarding end.) Though weak psychological egoism is a doctrine ultimately answerable to empirical evidence, we presently have excellent a priori reasons for accepting it and attempting to construct psychological theories that include it as an organizing principle. These (...) reasons have mainly to do with the idea that to understand the motivation behind an action, we need to understand the force of the consideration that motivates the agent, and the way to do this is to find a self-regarding end associated in the agent''s mind with acting on that consideration. (shrink)